Unify the two branches of process.c, with and without subprocesses.

 process.c: Reshuffle #include's.  Condition some of the global
 and static variables on `subprocesses'.
 (hold_keyboard_input, unhold_keyboard_input, kbd_on_hold_p):
 Leave only one implementation.
 (Fget_buffer_process, Fprocess_inherit_coding_system_flag)
 (kill_buffer_processes, Flist_system_processes)
 (Fprocess_attributes, init_process, syms_of_process): Unify the
 implementations for with subprocesses and without them.