Don't arbitrarily limit set_mark to certain chains

set_mark is not only allowed in PREROUTING or OUTPUT chain,
but also in custom chains, which are called from these.
MODULES-556: tcp_flags support for ip6tables

tcp_flags was listed as a supported feature for the ip6tables provider,
but it wasn't actually implemented
Merge pull request #442 from mhaskel/MODULES-556

MODULES-556: tcp_flags support for ip6tables
Merge pull request #427 from stesie/fix-set_mark-limit

Don't arbitrarily limit set_mark to certain chains