Fix scripted installation from media without local distfiles.

The bsdinstall script target did not have the infrastructure to fetch
distfiles from a remote server the way the interactive installer does
on e.g. bootonly media. Solve this by factoring out the parts of the
installer that deal with fetching missing distributions into a new
install stage called 'fetchmissingdists', which is called by both the
interactive and scripted installer frontends.

In the course of these changes, cleaned up a few other issues with
the fetching of missing distribution files and added a warning if
fetching the MANIFEST file, which is used to verify the integrity of
the distribution files. We should at some point add cryptographic
signatures to MANIFEST so that it can be fetched safely if not present
on the install media (which it is for bootonly media).

Initial patch by: Vinícius Zavam
PR: 255659, 250928
Reviewed by: dteske
MFC after: 4 weeks
Differential Revision: https://reviews.freebsd.org/D27121