Add rb_file_able_p function

[S-H-GAMELINKS]

Some function has similar code in file.c.
Add rb_file_able_p function and refactor these function.

[nobu]

How about return RBOOL(rb_eaccess(fname, R_OK) >= 0); using the macro defined now in range.c?

Moreover, \? *Qtrue *: *Qfalse and the reverse can be found over 200 places.

[S-H-GAMELINKS]

@nobu
Thanks you for comment.
That's good! I'll replace and using RBOOL macro.

[S-H-GAMELINKS]

@nobu

Moreover, ? *Qtrue *: *Qfalse and the reverse can be found over 200 places.

I'll try this another pull request.