pw(8): correct expiration period handling and command line overrides

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