git-cvsserver: Improved error handling for pserver

git-cvsserver: Improved error handling for pserver

  - Produce an error if the user tries to supply a password for anonymous

  - Clarify the error message produced when there's no [gitcvs.authdb]

  - Produce an E error if the authdb doesn't exist instead of spewing
    $! to the user

  - do crypt($user, descramble($pass)) eq $hash; crypt($user, $hash)
    eq $hash would accept any password

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>