Open the DAHDI pseudo device and set it to be nonblocking atomically

Open the DAHDI pseudo device and set it to be nonblocking atomically

Apparently on FreeBSD, attempting to set the O_NONBLOCKING flag separately
from opening the file was causing an "inappropriate ioctl for device" error.
While I cannot fathom why this would be happening, I certainly am not opposed
to making the code a bit more compact/efficient if it also fixes a bug.

(closes issue #14482)
Reported by: ys
Patches:
      meetme.patch uploaded by ys (license 281)
Tested by: ys

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@176249 65c4cc65-6c06-0410-ace0-fbb531ad65f3