core: Add a string list membership check function

This adds a pa_str_in_list() to check for a given string in a
space-separated list of strings. For now, this is merely present to
avoid duplication of role matching code (intended roles can be a
space-separate list) across modules.

src/pulsecore/core-util.h

@@ -204,6 +204,7 @@ void pa_unset_env_recorded(void);
 pa_bool_t pa_in_system_mode(void);
 
 #define pa_streq(a,b) (!strcmp((a),(b)))
+pa_bool_t pa_str_in_list_spaces(const char *needle, const char *haystack);
 
 char *pa_get_host_name_malloc(void);
 char *pa_get_user_name_malloc(void);