netfilter: cttimeout: allow to set/get default protocol timeouts

Default timeouts are currently set via proc/sysctl interface, the
typical pattern is a file name like:

/proc/sys/net/netfilter/nf_conntrack_PROTOCOL_timeout_STATE

This results in one entry per default protocol state timeout.
This patch simplifies this by allowing to set default protocol
timeouts via cttimeout netlink interface.

This should allow us to get rid of the existing proc/sysctl code
in the midterm.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>