Tests portability: umount(8)

1) Avoid hangs on unmounting NFS on NetBSD

NetBSD umount(8) on a NFS mount whose server is gone will wait forever
because umount(8) calls realpath(3) and tries to access the mount before
it calls unmount(2). The non-portable, NetBSD-specific umount -R flag
prevent that behavior.

We therefore introduce UMOUNT_F, defined as "umount -f" on Linux and
"umount -f -R" on NetBSD to take care of forced unmounts, especially
in the NFS case.

2) Enforce usage of force_umount wrapper with timeout

Whenever umount is used it should be wrapped in force_umount with
tiemout handling. That saves us timing issues, and it handles the
NetBSD NFS case.

3) Cleanup kernel cache flush.

We used (cd $M0 && umount $M0 )	as a portable kernel cache flush
trick, but it does not flush everything we need on Linux. Introduce
a drop_cache() shell function that reverts to previously used
echo 3 > /proc/sys/vm/drop_caches on Linux, and keeps
(cd $M0 && umount $M0 ) on other systems.

BUG: 1129939
Change-Id: Iab1f5a023405f1f7270c42b595573702ca1eb6f3
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/11114
Tested-by: NetBSD Build System <jenkins@build.gluster.org>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>

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