vt(4): Support syscons' SC_HISTORY_SIZE to configure history size

Therefore, to set histry size to 2000 lines, add the following line to
your kernel configuration file:
    options SC_HISTORY_SIZE=2000

The default history remains at 500 lines.

MFC of:		r274117
MFC r271320:

Use keys_add_int() where appropriate.  No functional changes.

Sponsored by:	The FreeBSD Foundation
MFC r273464:

Whitespace fixes.

Sponsored by:	The FreeBSD Foundation
MFC r273465:

Fix ctld(8) to not show the "auth-group <name> not assigned to any target"
warning for auth-groups assigned to a portal-group.

Sponsored by:	The FreeBSD Foundation
MFC r273466:

Remove spurious empty line.

Sponsored by:	The FreeBSD Foundation
MFC r273468:

Remove misleading statement.  Bump date.

Sponsored by:	The FreeBSD Foundation
MFC r273543:

Clean up (refactor) discovery a little; no functional changes.

Sponsored by:	The FreeBSD Foundation