libbb: introduce bb_signals and bb_signals_recursive,

which sets same handler for many signals. sig_catch is nuked
(bb_signals_recursive is more descriptive name).
*: use them as appropriate. 

function                                             old     new   delta
bb_signals_recursive                                   -      95     +95
bb_signals                                             -      52     +52
run_command                                          258     273     +15
svlogd_main                                         1368    1377      +9
runsv_main                                          1746    1752      +6
runsvdir_main                                       1643    1646      +3
UNSPEC_print                                          64      66      +2
time_main                                           1128    1127      -1
...
resize_main                                          246     210     -36
sig_catch                                             63       -     -63
set_fatal_sighandler                                  85      14     -71
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 5/24 up/down: 182/-548)        Total: -366 bytes

networking/sendmail.c

@@ -111,8 +111,10 @@ static void launch_helper(const char **argv)
 		_exit(127);
 	}
 	// parent - check whether child is alive
-	sig_catch(SIGCHLD, signal_handler);
-	sig_catch(SIGALRM, signal_handler);
+	bb_signals_recursive(0
+			+ (1 << SIGCHLD)
+			+ (1 << SIGALRM)
+			, signal_handler);
 	signal_handler(SIGCHLD);
 	// child seems OK -> parent goes on
 }