gcore: Make tilde-expanded filename visible.

gcore: Make tilde-expanded filename visible.

Most commands in GDB show the tilde-expanded filename in user visible
output.  This makes gcore behave the same.

Before:

  (gdb) generate-core-file ~/a/b
  Failed to open '~/a/b' for output.
  (gdb) generate-core-file ~/core
  Saved corefile ~/core

After:

  (gdb) generate-core-file ~/a/b
  Failed to open '/home/pedro/a/b' for output.
  (gdb) generate-core-file ~/core
  Saved corefile /home/pedro/core

Tested on x86_64 Fedora 17.

gdb/
2013-08-09  Pedro Alves  <palves@redhat.com>

	* gcore.c (create_gcore_bfd): Don't use tilde_expand here.
	(gcore_command): Use tilde_expand here, and when showing the
	filename to the user, show the expanded version.