Add functions Unix.timegm and Unix.timelocal

[Chris00]

No description provided.

[nojb]

Have you tried this one on Windows ? As far as I remember there is no timegm on Windows.

[dra27]

@nojb - you are correct, timegm and timelocal are both GNU/BSD extensions and are not present in any version of the Windows CRT

@Chris00 - timelocal is equivalent to mktime - the GNU docs seem to suggest its use should be rare - what's the argument here?

[xavierleroy]

Quoting the Linux man page:

"These functions are nonstandard GNU extensions that are also present on the BSDs. Avoid their use; see NOTES.

NOTES

The timelocal() function is equivalent to the POSIX standard function mktime(3). There is no reason to ever use it.

For a portable version of timegm(), set the TZ environment variable to UTC, call mktime(3) and restore the value of TZ. "

[Chris00]

I should have read more carefully the man page indeed. Would you mind that I add some documentation to the Unix module so that one does not have to look somewhere else?