parse-datetime: handle timezones reentrantly

This API change was prompted by a report by Pádraig Brady in:
https://bug.debian.org/851934#10
To help fix the bug, make parse_datetime2 more reentrant.
* NEWS: Document this incompatible change.
* lib/parse-datetime.h, lib/parse-datetime.y (parse_datetime2):
Add two arguments, the timezone and the timezone name.
All callers changed.  If TZ="..." is specified, use it for
calculating defaults.
* lib/parse-datetime.y: Don't include xalloc.h or use xmalloc, as
this code should be usable in a library.
(mktime_ok, get_effective_timezone):
Accept timezone arg too.  All callers changed.
(get_tz): Remove.
(get_effective_timezone): Check for failures.
* modules/parse-datetime: Add time_r, time_rz.  Remove xalloc.

modules/parse-datetime

@@ -20,9 +20,10 @@ setenv
 strftime
 unsetenv
 time
+time_r
+time_rz
 timegm
 verify
-xalloc
 
 configure.ac:
 gl_PARSE_DATETIME