pkg: allow multiple add arguments again

[kevans]

Summary

While pkg(7) add only handles a single 'add' argument, pkg-add(8) fully
handles multiple arguments.

Stop rejecting it, just turn off local-bootstrap mode and proceed to
remote bootstrap if we need it.

While we're here, check if the first argument to pkg add is even a pkg
package. If it's not, also do remote bootstrap instead. Future work
could improve this altogether by picking out a pkg package out of many
and local bootstrap then pass the rest through to the newly installed
pkg.

[kevans]

Fix the logic; the previous version would break if one tried to pkg add the
pkg directly produced by make package in ports-mgmt/pkg{,-devel}.

Use the more common variant of chopping off after the last '-'. This catches the
short-form of pkg-devel naturally, and we check for 'pkg.' if there was no
version delimiter to catch the short-form of pkg.