modules.pkgng.list_pkgs: fix `except` syntax

salt/modules/pkgng.py:297: [W0623(redefine-in-handler), list_pkgs] Redefining name 'ValueError' from builtins in exception handler
modules.freebsdpkg.list_pkgs: fix `except` syntax

salt/modules/freebsdpkg.py:241: [W0623(redefine-in-handler), list_pkgs] Redefining name 'ValueError' from builtins in exception handler
modules.freebsdpkg: rm unused import of 'os'

salt/modules/freebsdpkg.py:73: [W0611(unused-import), ] Unused import os
Merge pull request #7439 from hulu/freebsd-pkg-mgmt

fix pylint problems in rewritten FreeBSD package mgmt modules