preserve posix capabilities

preserve posix capabilities

POSIX capabilities are stored as security.capability xattr.
The --xattrs option alone won't store anything outside the
usernamespace, so we have to specifically ask for this
capability to be included when calling tar.
Note that we deliberately don't store the entire security
namespace as labeling by xattr is common with some security
modules and possibly a planned apparmor feature, too, so
this way we avoid restoring arbitrary lables from dumps and
templates we might not want.

Since these flags are used in two separate files I moved
them to @$PVE::LXC::COMMON_TAR_FLAGS;

The --acls flag for tar (and -A flag for rsync) have also
been added.