tsave: Make tilde-expanded filename visible.

tsave: Make tilde-expanded filename visible.

Before:

  (gdb) tsave ~/a/b
  Unable to open file '~/a/b' for saving trace data (No such file or directory)

After:

  (gdb) tsave ~/a/b
  Unable to open file '/home/pedro/a/b' for saving trace data (No such file or directory)

Tested on x86_64 Fedora 17.

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

	* tracepoint.c (tfile_start): Show tilde-expanded filename in
	error message.