Fix strptime decode for musl libc

POSIX does not define a %F for strptime[1], but does define %F for
strftime[2]. Afaik most libc's implement %F for both, but musl is very
standards-compliant and does not have %F on strptime, leading to
unparsed message tag times, which causes all backlog sent from bouncer
on startup to have a timestamp of the current time, instead of the
actual timestamp sent.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/strptime.html
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html

handle.c

@@ -91,7 +91,7 @@ static const time_t *tagTime(const struct Message *msg) {
 	static time_t time;
 	struct tm tm;
 	if (!msg->tags[TagTime]) return NULL;
-	if (!strptime(msg->tags[TagTime], "%FT%T", &tm)) return NULL;
+	if (!strptime(msg->tags[TagTime], "%Y-%m-%dT%T", &tm)) return NULL;
 	time = timegm(&tm);
 	return &time;
 }