res_pjsip: Strip spaces from items parsed from comma-separated lists

Configurations like "aors = a, b, c" were either ignoring everything after "a"
or trying to look up " b".  Same for mailboxes,  ciphers, contacts and a few
others.

To fix, all the strsep(&copy, ",") calls have been wrapped in ast_strip.  To
facilitate this, ast_strip, ast_skip_blanks and ast_skip_nonblanks were
updated to handle null pointers.

In some cases, an ast_strlen_zero() test was added to skip consecutive commas.

There was also an attempt to ast_free an ast_strdupa'd string in
ast_sip_for_each_aor which was causing a SEGV.  I removed it.

Although this issue was reported for realtime, the issue was in the res_pjsip
modules so all config mechanisms were affected.

ASTERISK-25829 #close
Reported-by: Mateusz Kowalski

Change-Id: I0b22a2cf22a7c1c50d4ecacbfa540155bec0e7a2

res/res_pjsip_notify.c

@@ -615,7 +615,7 @@ static int notify_endpoint(void *obj)
 
 	aors = ast_strdupa(data->endpoint->aors);
 
-	while ((aor_name = strsep(&aors, ","))) {
+	while ((aor_name = ast_strip(strsep(&aors, ",")))) {
 		RAII_VAR(struct ast_sip_aor *, aor,
 			 ast_sip_location_retrieve_aor(aor_name), ao2_cleanup);
 		RAII_VAR(struct ao2_container *, contacts, NULL, ao2_cleanup);