Add prefixcmp() (stol^H^H^H^Hborrowed from git)

This allows us to avoid the nasty repetition in strncmp(foo,
bar, strlen(foo)).  We'll miss out on the compiler optimizing
strlen() into sizeof() - 1 for string literals for this; but we
don't use this it for performance-critical functions anyways...

src/utils.h

@@ -103,4 +103,6 @@ void xpthread_mutex_destroy(pthread_mutex_t *mutex);
 
 void xpthread_cond_destroy(pthread_cond_t *cond);
 
+int prefixcmp(const char *str, const char *prefix);
+
 #endif