httpdate_to_datestamp() no longer needs to temporarily

httpdate_to_datestamp() no longer needs to temporarily
set the TZ environment variable to UTC, which is good because some systems
will fail to set it back afterwards.  Instead we grab the offset from the
global 'timezone' variable and add it to tm_sec.  mktime() will then
normalize the struct before converting to time_t.  This fixes a bug on some
systems where the time zone would appear to spontaneously switch to UTC.