BUrl: add a Redirect method

 * This takes a relative path as a parameter, and modifies the object to
point to the given location.
 * '..' is not handled yet, and will be sent as-is to the server.
 * Makes it possible to follow more types of 302 redirects

In particular, I can now run the tests from Opera's testsuite
(testsuite.opera.com), which shows I have more work to do on cookie
handling.
BDateTime: Time_T functions return or take a time_t

 * They used an unsigned int, which led to overflows when trying to set
them to a time before January 1st, 1970 (local time)
 * Some things use January 1st, 1970, GMT (or UTC) as a reference point.
In my timezone this leads to such a negative date. An example is cookie
expiration dates which are set to this date to expire them immediately.
Spotted by Opera testsuite.
 * This makes the method unuseable for dates after 2036 (signed 32-bit
time_t will overflow then. This gives us just 33 years to switch to a
64-bit time_t. In te meantime, please try using other methods to set the
date and time for BDateTime objects if you need to go this far.
HttpRequest: follow 302 redirects by default.
NetworkCookie: fix date validity check

 * January 1st 1970 is a perfectly valid expiration date for a cookie,
and shouldn't magically turn it into a session cookie.