* 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
c3358cbfc5