Fix PKGNG support in ZSH completions

Addresses #45

_bsd_pkg_pkgs still needs updating in ZSH itself.
Fully fix PKGNG support in ZSH by using custom _bsd_pkg_pkgs

Fixes #45

This will be upstreamed slightly differently eventually
Document fixes ZSH completions
Fix BASH completions with PKGNG
Use normal path lookup for pkg(8)
If PREFIX/sbin/pkg is not available, try using the ports-mgmt/pkg pkg-static version

This fixes build on 9.1, STABLE, and CURRENT. When ports-mgmt/pkg is
deinstalled, the PKG_BIN variable will return WRKDIR/pkg-static to
use to interact with the pkgng database. This is only a temporary
measure as the normal PREFIX/sbin/pkg will be used right away after
the install phase is called.

Fixes:	#46
Ensure that PORTSDIR/ports-mgmt/pkg exists before trying to use it

Addresses:	#46
2.4.10.3 Release
freshports support has been fixed