MFC r262957, r267591, r289634:

MFC r262957, r267591, r289634:

 r262957 (marcel):
  Change the terminal type/class for enabled serial lines to 3wire. This
  allows us to change the uart(4) driver to not hardcode specific line
  settings for the serial console.

  A terminal type of 3wire makes sure the console still works when no DCD
  signal is present, which preserves behviour. When it is known that the
  terminal server (or DCE in general) provides DCD, a terminal type/class
  of std can be used. This has the effect of being logged out when one
  disconnects from the console -- improving security overall.

 r267591 (grehan):
  Convert the potential console port over to using 3wire, for i386/amd64.

 r289634:
  Enable all callin ttys if the tty is an available console.

Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/10@290107 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f