Create new common/pathstuff.[ch]

This commit moves the path manipulation routines found on utils.c to a
new common/pathstuff.c, and updates the Makefile.in's accordingly.
The routines moved are "gdb_realpath", "gdb_realpath_keepfile" and
"gdb_abspath".

This will be needed because gdbserver will have to call "gdb_abspath"
on my next patch, which implements a way to expand the path of the
inferior provided by the user in order to allow specifying just the
binary name when starting gdbserver, like:

  $ gdbserver :1234 a.out

With the recent addition of the startup-with-shell feature on
gdbserver, this scenario doesn't work anymore if the user doesn't have
the current directory listed in the PATH variable.

I had to do a minor adjustment on "gdb_abspath" because we don't have
access to "tilde_expand" on gdbserver, so now the function is using
"gdb_tilde_expand" instead.  Otherwise, the code is the same.

Regression tested on the BuildBot, without regressions.

gdb/ChangeLog:
2018-02-28  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (COMMON_SFILES): Add "common/pathstuff.c".
	(HFILES_NO_SRCDIR): Add "common/pathstuff.h".
	* auto-load.c: Include "common/pathstuff.h".
	* common/common-def.h (current_directory): Move here.
	* common/gdb_tilde_expand.c (gdb_tilde_expand_up): New
	function.
	* common/gdb_tilde_expand.h (gdb_tilde_expand_up): New
	prototype.
	* common/pathstuff.c: New file.
	* common/pathstuff.h: New file.
	* compile/compile.c: Include "common/pathstuff.h".
	* defs.h (current_directory): Move to "common/common-defs.h".
	* dwarf2read.c: Include "common/pathstuff.h".
	* exec.c: Likewise.
	* guile/scm-safe-call.c: Likewise.
	* linux-thread-db.c: Likewise.
	* main.c: Likewise.
	* nto-tdep.c: Likewise.
	* objfiles.c: Likewise.
	* source.c: Likewise.
	* symtab.c: Likewise.
	* utils.c: Include "common/pathstuff.h".
	(gdb_realpath): Move to "common/pathstuff.c".
	(gdb_realpath_keepfile): Likewise.
	(gdb_abspath): Likewise.
	* utils.h (gdb_realpath): Move to "common/pathstuff.h".
	(gdb_realpath_keepfile): Likewise.
	(gdb_abspath): Likewise.

gdb/gdbserver/ChangeLog:
2018-02-28  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (SFILES): Add "$(srcdir)/common/pathstuff.c".
	(OBJS): Add "pathstuff.o".
	* server.c (current_directory): New global variable.
	(captured_main): Initialize "current_directory".

gdb/source.c

@@ -44,6 +44,7 @@
 #include "readline/readline.h"
 #include "common/enum-flags.h"
 #include <algorithm>
+#include "common/pathstuff.h"
 
 #define OPEN_MODE (O_RDONLY | O_BINARY)
 #define FDOPEN_MODE FOPEN_RB