Issue warning and refuse to proceed further if the configured

Issue warning and refuse to proceed further if the configured
repository signature_type is unsupported by bootstrap pkg(7).

Previously, when signature_type specified an unsupported method,
the bootstrap pkg(7) would proceed like when signature_type is
"none".  MITM attackers may be able to use this vulnerability and
bypass validation and install their own versions of pkg(8).

At this time, only fingerprint and none are supported by the
bootstrap pkg(7).

FreeBSD's official pkg(8) repository uses the fingerprint method
and is therefore unaffected.

Errata candidate.

Discussed with:	bapt@
Submitted by:	Fabian Keil
Obtained from:	ElectroBSD