Some users in Ubuntu/Debian seem to have a setup where all the
cgroup controllers are mounted on /sys/fs/cgroup rather than
any /sys/fs/cgroup/<controller> name. In the loop which detects
which controllers are present for a mount point we were modifying
'mnt_dir' field in the 'struct mntent' var, but not always restoring
the original value. This caused detection to break in the all-in-one
mount setup.

Fix that logic bug and add test case coverage for this mount
setup.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Clang does not like it when you pass a static variable to an
inline function

 vircgroupmock.c:462:22: error: static variable 'fakesysfsdir' is
  used in an inline function with external linkage [-Werror,-Wstatic-in-inline]

Just make the var non-static to avoid this
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

If a user cgroup name begins with "cgroup.", "_" or with any of
the controllers from /proc/cgroups followed by a dot, then they
need to be prefixed with a single underscore. eg if there is
an object "cpu.service", then this would end up as "_cpu.service"
in the cgroup filesystem tree, however, "waldo.service" would
stay "waldo.service", at least as long as nobody comes up with
a cgroup controller called "waldo".

Since we require a '.XXXX' suffix on all partitions, there is
no scope for clashing with the kernel 'tasks' and 'release_agent'
files.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

If a cgroup controller is co-mounted with another, eg

   /sys/fs/cgroup/cpu,cpuacct

Then it is a requirement that there exist symlinks at

   /sys/fs/cgroup/cpu
   /sys/fs/cgroup/cpuacct

pointing to the real mount point. Add support to virCgroupPtr
to detect and track these symlinks
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Some aspects of the cgroups setup / detection code are quite subtle
and easy to break. It would greatly benefit from unit testing, but
this is difficult because the test suite won't have privileges to
play around with cgroups. The solution is to use monkey patching
via LD_PRELOAD to override the fopen, open, mkdir, access functions
to redirect access of cgroups files to some magic stubs in the
test suite.

Using this we provide custom content for the /proc/cgroup and
/proc/self/mounts files which report a fixed cgroup setup. We
then override open/mkdir/access so that access to the cgroups
filesystem gets redirected into files in a temporary directory
tree in the test suite build dir.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>