MT-unsafe time modules

[Bruno Haible <bruno@clisp.org>]

When a module is under LGPL, it is implicitly advertised as suitable for
libraries. And libraries should be MT-safe (for 20 years already). So,
such modules should be MT-safe.

Looking at the LGPLed time-related modules:

* This one is already MT-safe:

timespec

* These need to be made MT-safe:

strftime-fixes
wcsftime
ctime
localtime

mktime, mktime-internal
timegm

time_r

strptime

time_rz
c-nstrftime, nstrftime
parse-datetime, parse-datetime2


I will try to tackle them, in roughly the given order.

Bruno