Introduce gdb_tilde_expand

Introduce gdb_tilde_expand

Currently, whenever we want to handle paths provided by the user and
perform tilde expansion on GDB, we rely on "tilde_expand", which comes
from readline.  This was enough for our use cases so far, but the
situation will change when we start dealing with paths on gdbserver as
well, which is what the next patches implement.

Unfortunately it is not possible to use "tilde_expand" in this case
because gdbserver doesn't use readline.  For that reason I decided to
implement a new "gdb_tilde_expand" function, which is basically a
wrapper for "glob" and its GNU extension, GLOB_TILDE_CHECK.  With the
import of the "glob" module from gnulib, we're sure that "glob" always
supports this extension.

gdb/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (SFILES): Add gdb_tilde_expand.c.
	(HFILES_NO_SRCDIR): Add gdb_tilde_expand.h.
	(COMMON_OBS): Add gdb_tilde_expand.o.
	* common/gdb_tilde_expand.c: New file.
	* common/gdb_tilde_expand.h: Likewise.

gdb/gdbserver/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (SFILES): Add $(srcdir)/common/gdb_tilde_expand.c.
	(OBS): Add gdb_tilde_expand.o.