Add xgetc, xfgets, xputc and xputs.

Some declarations are modified in line with other functions in xfile.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #4 from omasanori/missing-parts

Add xgetc, xfgets, xputc and xputs.
don't use adjacent underscores
delete xf_unpack
accurate error handling on xfgetc and xfputc
error handling in xfgets