Add -f support to 'pkg bootstrap' and 'pkg add' to force installation
of pkg(8) even if already installed.

This is useful if you somehow messup pkg(8) and need to reinstall from
remote with it already being registered in the pkg(8) /var/db/pkg database.

Also add some sanity checks to 'pkg add'.

Approved by:	bapt
MFC after:	2 days