pkg: allow multiple add arguments again

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.

Reviewed by: bapt, manu (earlier version)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28766