Support Last-Modified behind proxies which return UTC instead of GMT.

The standard states that GMT must be used, but that UTC is equivalent. Still
parse UTC as otherwise this causes problems for pkg(8). It will refetch
the repository every time 'pkg update' or other remote operations
are used behind these proxies.

RFC2616: "All HTTP date/time stamps MUST be represented in Greenwich Mean
Time (GMT), without exception. For the purposes of HTTP, GMT is exactly equal
to UTC (Coordinated Universal Time).""

Approved by: bapt (mentor)
Reviewed by: des, peter
Sponsored by: EMC / Isilon Storage Division
MFC after: 1 week