small refactor in mirb

Change the position of -L option from MRUBY_LIBS to MRUBY_LDFLAGS

remove unused variable

fix mruby command returning EXIT_SUCCESS when program file can't be opened.

GLOBAL_RAND_SEED_KEY is no longer used.

opened.

fix argument spec setting of Kernel.exit method.

this method requires 1 optional argument.

Add existing include/ directory only.

Fix #1803

Add mruby-bin-mruby-config

mruby command should not execute program on "syntax only option(-c)".

fix indent.

fix off-by-one error in attrsym

The allocation size for null-terminated character is lacking.
In actual, it is no problem in most case
because codegen_palloc allocates more a memory size than required size.

Add notes about new features.

Fix #1793

fix off-by-one error in String#rindex(fixnum)

fix a comment end of include guard

null-terminated string should not be included in search targets.