Use vfork() instead of fork() if available

Hopefully this reduces the chance of hitting ‘unable to fork: Cannot
allocate memory’ errors.  vfork() is used for everything except
starting builders.

src/libutil/util.hh

@@ -266,6 +266,9 @@ void closeOnExec(int fd);
    sanitize file handles 0, 1 and 2. */
 void setuidCleanup();
 
+/* Call vfork() if available, otherwise fork(). */
+extern pid_t (*maybeVfork)();
+
 
 /* User interruption. */