MINOR: config: make MAX_PROCS configurable at build time

For some embedded systems, it's pointless to have 32- or even 64- large
arrays of processes when it's known that much fewer processes will be
used in the worst case. Let's introduce this MAX_PROCS define which
contains the highest number of processes allowed to run at once. It
still defaults to LONGBITS but may be lowered.
diff --git a/src/cli.c b/src/cli.c
index d1d1e96..3c2a55d 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -359,7 +359,7 @@
 				set = 0;
 				break;
 			}
-			if (parse_process_number(args[cur_arg], &set, LONGBITS, NULL, err)) {
+			if (parse_process_number(args[cur_arg], &set, MAX_PROCS, NULL, err)) {
 				memprintf(err, "'%s %s' : %s", args[0], args[1], *err);
 				return -1;
 			}