core: use GUnixSignalWatchSource to simplify signal handling

Replace the pthread_sigwait()-based signal handling with
g_unix_signal_add()-based handling, and get rid of all the
now-unnecessary calls to nm_unblock_posix_signals() when spawning
subprocesses.

As a bonus, this also fixes the "^C in gdb kills NM too" bug.

src/main-utils.h

@@ -23,7 +23,7 @@
 
 #include <glib.h>
 
-gboolean nm_main_utils_setup_signals (GMainLoop *main_loop, gboolean *quit_early_ptr);
+void     nm_main_utils_setup_signals (GMainLoop *main_loop);
 
 gboolean nm_main_utils_write_pidfile (const char *pidfile);