MFC r282645, r282646 and r282730:

MFC r282645, r282646 and r282730:
* Prevent switching to NULL or own window in the "vt_proc_window_switch"
  function. This fixes an issue where X11 keyboard input can appear
  stuck. The cause of the problem is a duplicate TTY device window
  switch IOCTL during boot, which leaves the "vt_switch_timer" running,
  because the current window is already selected. While at it factor out
  some NULL checks.
* The "SYSCTL_INT()" default value is only used for read only SYSCTLs
  and is not applicable unless the integer pointer is NULL. Set it to
  zero to avoid confusion. While at it remove extra semicolon at the end
  of the "VT_SYSCTL_INT()" macro.
* Ensure the result from signed subtraction under modulus does not
  become negative.

PR:			200032

git-svn-id: svn://svn.freebsd.org/base/stable/10@283187 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f