Improved error management in File.new (Errno can be undefined)

Conflicts:
	src/io.c

 - refs https://github.com/mruby/mruby/commit/36e234aa377d50d8ee425c7868e0651cf78e85cf

Support mrb_module_get

When system file descriptor table is full, errno is set to
ENFILE instead of EMFILE.  They are similar but different.

Truncate the file to zero length on opening when "w" mode is set

closes #3.

EAGAIN indicates the system is under heavy load.
Retrying make things worse.

Catch up with API changes on mrb_intern() -> mrb_intern_cstr()

closes #1