gef-remote: Partially reverting changes introduced by 9dce8bffcaa008e8822ec51d998eacc244731821
gef-remote: Partially reverting changes introduced by 9dce8bffcaa008e8822ec51d998eacc244731821

The problem is that some "old" versions of gdbserver will return systematically
0 when using gdb.selected_inferior().pid, therefore making `get_pid()` fail when
used in remote mode (non-extended). Since adding the PID is a minor option, and
easy to get, I revert back to enforcing the option `-p` on gef-remote in
non-extended mode.

wmliang/gef,hugsy/gef,hugsy/gef,hugsy/gef,wmliang/gef