[clock format] doesn't support locales

[clock format] always returns day and month names in
Engish, although it uses strftime() internally which
supports locales, but only if the LC_TIME locale
category has been initialized from the respective
environment variable(s) before.

I first thought of adding a call to
setlocale(LC_TIME,"") or setlocsl(LC_ALL, "") to
unix/tclUnixInit.c, but that would have meant, that a
script can't change the locale it inherited from it's
parent process.
This in turn broke many of the tests in clock.test when
"make test" was called from within a German locale.

Therefore I added the setlocale(LC_TIME,"") call to
tclUnixTime.c just before strftime() is being called,
so that changing env(LC_CTIME) from within a script
changes gets reflected in [clock format] immediately.