VT_WAITACTIVE: Avoid returning EINTR when not necessary

We should generally prefer to return ERESTARTNOHAND rather than EINTR,
so that processes with unhandled signals that get ignored don't return
EINTR.

This can help with X startup issues:

    Fatal server error:
    xf86OpenConsole: VT_WAITACTIVE failed: Interrupted system call

although the real fix is having the X server always retry EINTR
regardless (since EINTR does happen for signals that have handlers
installed). Keithp has a patch for that.

Regardless, ERESTARTNOHAND is the correct thing to use.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>