MFC r326738: pw(8): correct expiration period handling

MFC r326738: 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)
Approved by:	mav (mentor)
Relnotes:	yes

git-svn-id: svn://svn.freebsd.org/base/stable/10@326849 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f