Fix unveil(2) in patch(1) with explicit patchfile.

Fix unveil(2) in patch(1) with explicit patchfile.

A backup file should be created in the directory of the original
file, but only the current directory was unveiled.  Then the patched
file was created in /tmp and did not replace the original patchfile
in place.  If a patchfile is passed in argv[0], unveil its directory
instead of current directory.

OK florian@ deraadt@ millert@