pkg-bootstrap: complain on improper pkg bootstrap usage

Right now, the bootstrap will gloss over things like pkg bootstrap -x or
pkg bootstrap -f pkg. Make it more clear that this is incorrect, and hint
at the correct formatting.

Reported by: jhb (IIRC via IRC)
Approved by: bapt, jhb, manu
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24750