libc: Add "Z" as TZ designator for strptime.

ISO 8601 allows use of "Z" as the time zone designator. Update the
strptime parser to allow this usage.

While we are at it, update the manpage to reflect that both UTC and Z
are now valid options.

Reviewed by:	des
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D53083

lib/libc/stdtime/strptime.3

@@ -171,7 +171,7 @@ is taken as noon.
 The
 .Fa %Z
 format specifier only accepts time zone abbreviations of the local time zone,
-or the value "GMT".
+and the values "GMT", "UTC", or "Z".
 This limitation is because of ambiguity due to of the over loading of time
 zone abbreviations.
 One such example is

lib/libc/stdtime/strptime.c

@@ -546,7 +546,8 @@ _strptime(const char *buf, const char *fmt, struct tm *tm, int *GMTp,
 				zonestr[cp - buf] = '\0';
 				tzset();
 				if (0 == strcmp(zonestr, "GMT") ||
-				    0 == strcmp(zonestr, "UTC")) {
+				    0 == strcmp(zonestr, "UTC") ||
+				    0 == strcmp(zonestr, "Z")) {
 				    *GMTp = 1;
 				} else if (0 == strcmp(zonestr, tzname[0])) {
 				    tm->tm_isdst = 0;