Use target_continue{,_no_signal} instead of target_resume

This commit implements a new function, target_continue, on top of the
target_resume function.  Then, it replaces all calls to target_resume
by calls to target_continue or to the already existing
target_continue_no_signal.

This is one of the (many) necessary steps needed to consolidate the
target interface between GDB and gdbserver.  In particular, I am
interested in the impact this change will have on the unification of
the fork_inferior function (which I have been working on).

Tested on the BuildBot, no regressions introduced.

gdb/gdbserver/ChangeLog:
2016-09-31  Sergio Durigan Junior  <sergiodj@redhat.com>

	* server.c (start_inferior): New variable 'ptid'.  Replace calls
	to the_target->resume by target_continue{,_no_signal}, depending
	on the case.
	* target.c (target_stop_and_wait): Call target_continue_no_signal
	instead of the_target->resume.
	(target_continue): New function.

gdb/ChangeLog:
2016-09-31  Sergio Durigan Junior  <sergiodj@redhat.com>

	* fork-child.c (startup_inferior): Replace calls to target_resume
	by target_continue{,_no_signal}, depending on the case.
	* linux-nat.c (cleanup_target_stop): Call
	target_continue_no_signal instead of target_resume.
	* procfs.c (procfs_wait): Likewise.
	* target.c (target_continue): New function.
	* target/target.h (target_continue): New prototype.

gdb/linux-nat.c

@@ -4289,7 +4289,7 @@ cleanup_target_stop (void *arg)
   gdb_assert (arg != NULL);
 
   /* Unpause all */
-  target_resume (*ptid, 0, GDB_SIGNAL_0);
+  target_continue_no_signal (*ptid);
 }
 
 static VEC(static_tracepoint_marker_p) *