save breakpoints: Make tilde-expanded filename visible.

save breakpoints: Make tilde-expanded filename visible.

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

Before:

  (gdb) save breakpoints ~/a/b
  Unable to open file '~/a/b' for saving (No such file or directory)

After:

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

Tested on x86_64 Fedora 17.

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

        * breakpoint.c (save_breakpoints): Show tilde-expanded filename in
	error message.