pkg(7): add an -r reponame option for bootstrap and add

This is limited to bootstrap/add because some real pkg(8) commands
have -r flags with an incompatible meaning/usage, e.g., pkg-audit.
pkg(7) will still commence the search as it has, but it will ignore any
repo objects without the given name so that overrides and whatnot still
work as expected.

The use of it for add is noted in the manpage; notably, that the
signature config for that repository will be used over global config if
it's specified. i.e., pkg(7) should assume that the given pkg did come
from that repository and treat it appropriately.

Reviewed by: bapt, manu
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28524