netfilter: print the list of register loggers

This patch modifies the proc output to add display of registered
loggers. The content of /proc/net/netfilter/nf_log is modified. Instead
of displaying a protocol per line with format:
	proto:logger
it now displays:
	proto:logger (comma_separated_list_of_loggers)
NONE is used as keyword if no logger is used.

Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: Patrick McHardy <kaber@trash.net>