POSIX: have sysconf(_SC_OPEN_MAX) return the rlimit value.

* we use this value in posix_spawn_file_actions_adddup2() for instance, ninja then
fails with "Bad file descriptor".

src/system/libroot/posix/unistd/conf.cpp

@@ -37,7 +37,7 @@ getdtablesize(void)
 {
 	struct rlimit rlimit;
 	if (getrlimit(RLIMIT_NOFILE, &rlimit) < 0)
-		return 0;
+		return OPEN_MAX;
 
 	return rlimit.rlim_cur;
 }
@@ -74,7 +74,7 @@ __sysconf(int name)
 		case _SC_NGROUPS_MAX:
 			return NGROUPS_MAX;
 		case _SC_OPEN_MAX:
-			return OPEN_MAX;
+			return getdtablesize();
 		case _SC_SAVED_IDS:
 			return 1;
 		case _SC_STREAM_MAX: