Fix segfaults caused by faulty command parsing

This patch fixes faulty command parsing introduced by
f0f5de9a9e. When that commit allowed
criteria reset on ';' delimeters in commands lists, it failed to account
for its inner ','-parsing loop eating threw the entire rest of the
string.

This patch refactors argsep to use a list of multiple separators, and
(optionally) return the separator that it matched against in this
iteration via a pointer. This allows it to hint at the command parser
which separator was used at the end of the last command, allowing it to
trigger a potential secondary read of the criteria.

Fixes #4239

include/stringop.h

@@ -24,6 +24,6 @@ int unescape_string(char *string);
 char *join_args(char **argv, int argc);
 
 // Split string into 2 by delim, handle quotes
-char *argsep(char **stringp, const char *delim);
+char *argsep(char **stringp, const char *delim, char *matched_delim);
 
 #endif