Add vt(4) support to the console initialisation script, specifically:

- Identify the console driver used and print syscons or vt as appropriate.

- If vt is used and a keymap could not be loaded, then try to replace the
  keymap name configured in rc.conf based on a replacement list in this
  script. Warn about the fact, that a syscons keyname is configured and
  report the replacement used under vt.

- If no replacement keymap is found, no keymap is loaded and a warning
  is displayed, which points at the conversion script and allows the
  conversion of keymaps not part of the official distribution.

This patch has been sent to the -hackers list for review, but no comment
has been received, yet. It is tested to work under syscons and vt on my
system (on vt with either the syscons or vt keymap file name in rc.conf).

MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270935 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f