switch-on-connect: Add blacklisting

Add a new module argument, blacklist, which is a regular expression.
If the sink/source name matches the provided blacklist regex, don't
automatically switch to it. By default, no devices are blacklisted.

Add a new function to check whenever a regex pattern is valid, plus
extra NULL asserts in pa_match.

src/pulsecore/core-util.h

@@ -158,6 +158,7 @@ size_t pa_vsnprintf(char *str, size_t size, const char *format, va_list ap);
 char *pa_truncate_utf8(char *c, size_t l);
 
 int pa_match(const char *expr, const char *v);
+bool pa_is_regex_valid(const char *expr);
 
 char *pa_getcwd(void);
 char *pa_make_path_absolute(const char *p);