pkg-(add|install|upgrade): Respect locked packages on forced invocation.

This re-establishes pkg 1.2 behaviour.

Beforehand:
  pkg lock -y zip
  Locking zip-2.9
  pkg install -fy zip
  Updating repository catalogue
  FreeBSD repository is up-to-date
  All repositories are up-to-date
  Checking integrity... done (0 conflicting)
  The following 1 packages will be affected (of 0 checked):

  Installed packages LOCKED:
        Package zip-2.9 is locked and may not be upgraded to version 3.0

  [pkgtest] [1/1] Upgrading zip from 2.9 to 3.0: 100%

Afterwards:
  pkg lock -y zip
  Locking zip-2.9
  pkg install -fy zip
  Updating repository catalogue
  FreeBSD repository is up-to-date
  All repositories are up-to-date
  Updating database digests format: 100%

  zip-2.9 is locked and may not be modified
  Checking integrity... done (0 conflicting)
  The most recent version of packages are already installed

Same for "pkg add" and "pkg upgrade".