[MERGE] Rename enable-linux-netfilter to enable-nf-transparent

I'd like to propose the attached patch, to rename the build option
--enable-linux-netfilter to --enable-nf-transparent. This is for 2
reasons:

1. It is consistent with the remainder of the transparent proxy options
(ifpw-transparent, ipf-transparent, pf-transparent).

2. It causes less confusion with my proposed netfilter marking patch,
which also relies on netfilter libraries, but different ones.
--enable-linux-netfilter implies the whole of the netfilter libraries
are being included, when in actual fact it is only one for the purposes
of transparent proxying.

Netfilter marking patch to follow soon...

Regards,

Andy