Per review, Rusty noted a preference for the current behvaior, i.e. always
doing a login if p4login is called.
This version introduces SDP_ALWAYS_LOGIN, which defaults to 1 (enabled),
which preserves the traditional 'always login each time the script is
run' behavior, while allowing the new 'login only if a ticket is not
available' behavior. (An advantage of the traditional behavior is that
it fixes the case where a human admin does a 'p4 login' but forgets the
'-a' on a server a network card configuration setup such that 'p4 login -a'
Customers desiring the 'login only if required' can set SDP_ALWAYS_LOGIN=0