check_disk: no longer hangs on hanging filesystems

Just turning attached patch of github issue #867 into a push request.

"Hi,
i created a patch for check_disk (v2025/1.4.13) which can handle hanging nfs filesystems. Imagine you mounted a share from a NAS at the mountpoint /mnt. Now if the Storage device or whatever acts as NFS server dies or encounters a network problem, you will see messages like "NFS server nas.naprax.de not responding still trying" and every process accessing files inside the /mnt directory will be blocked, maybe forever. Depending on the mount options the hanging processes may even be invulnerable to a kill -9. This also applies to check_disk. If you have a service monitoring usage of /mnt with "check_disk ... -p /mnt", it will also be blocked. Nagios will report a timeout then. But the bad thing is, every minutes another check_disk will be started which also will hang then. Sooner or later your process list fills up with unkillable check_disks.
The critical piece of code inside check_disk is the stat system call, which is in the moment needed to find out, if a path exists at all. If that stat call hits a directory which is mounted from a dead nfs server, it will not return with an error code, but will not return at all.
I found out that although processes cannot be killed in such a situations, threads can. So i rewrote the stat_path subroutine in a way, where the critical stat is executed in it's own thread. If this thread does not terminate within the --timeout interval, it is considered to be blocked by a dead nfs filesystem and the thread will be detached.
I tested it on Linux 2.6.18 (gcc 4.1.2) and Solaris 10/x86 (gcc 3.4.3)"