CURLOPT_FOLLOWLOCATION: add a CURLFOLLOW_NO_CUSTOMREQUEST bit

With this change, the argument passed to the CURLOPT_FOLLOWLOCATION
option is treated as a bitmask instead of just a long. If the new
CURLFOLLOW_NO_CUSTOMREQUEST bit is set in the bitmask, it means that
libcurl will NOT allow a custom method override the HTTP request method
after a redirect is followed. As is otherwise the default behavior (that
surprises many users).

This change is forward compatible because CURLOPT_FOLLOWLOCATION has
been documented to accept the exact value of '1' to enable redirect
following and therefore the other bits were left unused and
undefined. We now add value to another bit. Starting in 7.66.0, the
value 1 and the first bit still enables plain redirect following but the
second bit adds more meaning.

This change is backward compatible in the following way: setting the
CURLFOLLOW_NO_CUSTOMREQUEST bit in a program that still uses an older
libcurl installation at run-tim will have no effect. This is because
older libcurl code checked if the value was non-zero and then enabled
redirect following. Of course older libcurl will always let the set
CURLOPT_CUSTOMREQUEST string override the method, disregarding what the
HTTP response code suggests.

Test 1563 added to verify the functionality.