Fix bug in mruby-test gem (fix #3094)

Add more limitations

  - defined?
  - alias on global variables
  - Operator modification
  - Kernel.binding missing

In an used build path mruby-test wasn't updating mrbtest.c in the
case that the mgem selection was changed. This lead to:
  - a missing reference in case a GEM was removed
  - ignoring all new GEMs added to the build configuration

This fix keeps track of the active gems and demands a rebuild of
mrbtest.c in case that the gem selection changed.

Add toolchain support for OpenWRT

Addition to mruby limitation documentation

Limitations

Fix missing dependency on mruby-print to support 'puts'

Add String#rjust to mruby-string-ext

Fix a typo

su ported ->
supported
  ^

protect NoMethodError from calling to_hash in replace

Fix SEGV by stack extension in mrb_get_args()

mrb_get_args() keeps pointer of the current stack. But address of the
current stack maybe changed by method call.

'i' format character calls #to_i when the argument isn't integer but
has #to_i.

Here is a code that may call #to_i in mrb_get_args():

    case 'i':
      // ...
            default:
              *p = mrb_fixnum(mrb_Integer(mrb, ARGV[arg_i]));
              break;
     // ...

Here is a code #to_i is called:

    class X
      def initialize(i)
        @i = i
      end

      def to_i
        @i
      end
    end

    [][X.new(0), 0] # X#to_i is called

So, mrb_get_args() shouldn't keep pointer and use it. mrb_get_args()
should always refer mrb->ci->stack to use valid address of the current
stack.

Small grammar fix

[ci skip]

Fix SEGV on re-raising NoMemoryError

Think about the following Ruby script:

segv.rb:

    begin
      lambda do
        lambda do
          "x" * 1000 # NoMemoryError
        end.call
      end.call
    rescue
      raise
    end

If memory can't allocate after `"x" * 1000`, mruby crashes.

Because L_RAISE: block in mrb_vm_exec() calls mrb_env_unshare() via
cipop() and mrb_env_unshare() uses allocated memory without NULL check:

L_RAISE: block:

    L_RAISE:
      // ...
      while (ci[0].ridx == ci[-1].ridx) {
        cipop(mrb);
        // ...
      }

cipop():

    static void
    cipop(mrb_state *mrb)
    {
      struct mrb_context *c = mrb->c;

      if (c->ci->env) {
        mrb_env_unshare(mrb, c->ci->env);
      }

      c->ci--;
    }

mrb_env_unshare():

    MRB_API void
    mrb_env_unshare(mrb_state *mrb, struct REnv *e)
    {
      size_t len = (size_t)MRB_ENV_STACK_LEN(e);
      // p is NULL in this case
      mrb_value *p = (mrb_value *)mrb_malloc(mrb, sizeof(mrb_value)*len);

      MRB_ENV_UNSHARE_STACK(e);
      if (len > 0) {
        stack_copy(p, e->stack, len); // p is NULL but used. It causes SEGV.
      }
      e->stack = p;
      mrb_write_barrier(mrb, (struct RBasic *)e);
    }

To solve the SEGV, this change always raises NoMemoryError even when
realloc() is failed after the first NoMemoryError in
mrb_realloc(). mrb_unv_unshare() doesn't need to check NULL with this
change.

But it causes infinite loop in the following while:

    L_RAISE:
      // ...
      while (ci[0].ridx == ci[-1].ridx) {
        cipop(mrb);
        // ...
      }

Because cipop() never pops ci.

This change includes cipop() change. The change pops ci even when
mrb_unv_unshare() is failed by NoMemoryError.

This case can be reproduced by the following program:

    #include <stdlib.h>
    #include <mruby.h>
    #include <mruby/compile.h>

    static void *
    allocf(mrb_state *mrb, void *ptr, size_t size, void *ud)
    {
      static mrb_bool always_fail = FALSE;

      if (size == 1001) {
        always_fail = TRUE;
      }
      if (always_fail) {
        return NULL;
      }

      if (size == 0) {
        free(ptr);
        return NULL;
      } else {
        return realloc(ptr, size);
      }
    }

    int
    main(int argc, char **argv)
    {
      mrb_state *mrb;
      mrbc_context *c;
      FILE *file;

      mrb = mrb_open_allocf(allocf, NULL);
      c = mrbc_context_new(mrb);
      file = fopen(argv[1], "r");
      mrb_load_file_cxt(mrb, file, c);
      fclose(file);
      mrbc_context_free(mrb, c);
      mrb_close(mrb);

      return EXIT_SUCCESS;
    }

Try the following command lines:

    % cc -I include -L build/host/lib -O0 -g3 -o no-memory no-memory.c -lmruby -lm
    % ./no-memory segv.rb

Update license year range to 2016

Fix passing all zero string to Kernel#Integer