pw(8): correct expiration period handling and command line overrides
to preconfigured values for -e, -p and -w flags.

Use non-negative symbols instead of magic values
in passwd_val/pw_password functions.

PR: 223431
Submitted by: Yuri Pankov (in part, patch for the manual)
Reported by: mav (mentor)
MFC after: 3 days
Relnotes: yes