Provide a friendlier message when bootstrap fails due to address resolution

[thj]

Summary

The current message when bootstapping pkg fails for any reason implies that pkg
is not available.

$ pkg bootstrap
The package management tool is not yet installed on your system.
Do you want to fetch and install it now? [y/N]: y
Bootstrapping pkg from pkg+http://pkg.FreeBSD.org/FreeBSD:12:amd64/quarterly, please wait...
pkg: Error fetching http://pkg.FreeBSD.org/FreeBSD:12:amd64/quarterly/Latest/pkg.txz: No address record
A pre-built version of pkg could not be found for your system.
Consider changing PACKAGESITE or installing it from ports: 'ports-mgmt/pkg'.

When bootstrap fails because DNS is either not configured or working this is
pretty scary. Offer a friendlier message when bootstrap fails due to name
resolution failing.

[bz]

usr.sbin/pkg/pkg.c

919

this misses a '.' at the end.