build: enable fusermount by default

The fusermount available in gluster is customized to ensure
mounting with SELinux happens properly, i.e - to have a separate
thread for fuse_thread_proc which can process getxattr requests
and in parallel perform sys_mount() in a different thread, thereby
avoiding a deadlock.

However our build and packaging defaults to not including our
fusermount. This patch reverses the defaults.

Backport from:
> Change-Id: I793af4c2f56aeac46efae3db30e7c64ee7c18850
> BUG: 811217
> Signed-off-by: Anand Avati <avati@redhat.com>
> Reviewed-on: http://review.gluster.org/4773
> Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
> Tested-by: Gluster Build System <jenkins@build.gluster.com>

Change-Id: If5fb368b6737fe8b6f22d2f2aa0785baa7f96364
BUG: 1058666
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/6831
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Justin Clift <justin@gluster.org>
Tested-by: Justin Clift <justin@gluster.org>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
build: really disable fusermount if you say so

There is no logic in configure.ac that provides a $disable_fusermount
variable. So, use the $enable_fusermount variable instead.

Backport from:
> Follow-up-for: http://review.gluster.org/4773
> Change-Id: I81cdbd0045409d0036438d542ca6dc1934f784e4
> BUG: 948205
> Signed-off-by: Niels de Vos <ndevos@redhat.com>
> Reviewed-on: http://review.gluster.org/4803
> Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
> Reviewed-by: Sachidananda Urs <sacchi@gmail.com>
> Tested-by: Gluster Build System <jenkins@build.gluster.com>

Change-Id: Ie5fa36fa6cf40336e75efbf7fd8527c3ff466e4c
BUG: 1058666
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/6832
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Justin Clift <justin@gluster.org>
Tested-by: Justin Clift <justin@gluster.org>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>