gdb: add setter/getter for inferior cwd

gdb: add setter/getter for inferior cwd

Add cwd/set_cwd to the inferior class, remove set_inferior_args.
Keep get_inferior_args, because it is used from fork_inferior, in shared
code.  The cwd could eventually be passed as a parameter eventually,
though, I think that would be cleaner.

Change-Id: Ifb72ea865d7e6f9a491308f0d5c1595579d8427e