x11: cache atom names in vdagent_x11_get_atom_name()
XGetAtomName() returns a new string that should be freed, when no longer needed, but the current impl doesn't do that.
To preserve the ease of use for debugging purposes, cache the atom names and free them when the cache is full and when the x11 struct is destroyed.
Fixes: #22 (closed)
Signed-off-by: Jakub Janků jjanku@redhat.com