Prevent unintended file desriptor leak in grantpt.

The pt_chown program is completely transparently called.  It might
not be able to live with the various file descriptors the program
has open at the time of the call (e.g., under SELinux).  Close all
but the needed descriptor and connect stdin, stdout, and stderr
with /dev/null.  pt_chown shouldn't print anything when called to
do real work.

(cherry picked from commit 139ee080)