Rename P4#input to P4#input=, and fix all pre-existing calls. The
main existing callers are p4.run_password, p4.run_submit and
and p4.run_login. There was also a bug with p4.run_password() which
meant that you couldn't change a user's password if they hadn't
already set one. Fixed that while I was there and added a test
case to check that password setting, use and clearing works.