generic/079: make sure users 'nobody' and 'daemon' exist

generic/079: make sure users 'nobody' and 'daemon' exist

Test 'src/t_immutable' assumes the existence of users 'nobody' and
'daemon' on the system.  If any of these users don't exist, the test will
fail with:

  add_acl(/media/scratch/079/immutable.f) did not set errno == EPERM
  ...

because it tries to set ACLs such as:

  u::rwx,g::rwx,o::rwx,u:daemon:rwx,m::rwx

Use the new _require_user_exists() function to ensure these users exist.

Signed-off-by: Luis Henriques <lhenriques@suse.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>