time in rtc 822

check_http prints the expiry time of a certificate in an ambiguous fixed format
this patch changes that to rfc 822 format (see: man strftime)
e.g.:
$ LC_ALL=C ./plugins/check_http -H www.verisign.com -C 14
OK - Certificate will expire on Fri, 25 Jun 2012 23:59:00 +0000.
and another example:
$ LC_TIME=nl_NL ./plugins/check_http -H www.verisign.com -C 14
OK - Certificate will expire on vr, 25 jun 2012 23:59:00 +0000.

This patch was dedicated by mmjo@users.sourceforge.net