gdb: make progspace::exec_filename private, add getter / setter

Just like the title says... I think this makes things a bit clearer, for
instance where the exec filename is set.  It also makes the read call
sites a bit nicer, avoiding the `.get ()`.

Change-Id: If8b58ae8f6270c8a34b868f6ca06128c6671ea3c
Approved-By: Tom Tromey <tom@tromey.com>