lookup: Move UDP/TCP port and Ethernet type lookup into own module

Up to now, the lookup of TCP/UDP port names and Ethernet types was
tightly integrated with the dissector infrastructure, since it is its
main user. However, flowtop also makes use of the name lookup
functionality without needing the actual dissector infrastructure. Thus,
the basic dissector infrastructure also needs to be linked into flowtop
without actually being used.

Fix this by extracting the port/ethertype lookup into an own module
which can then be used either directly (for flowtop) or as part of the
dissector infrastructure (for netsniff-ng).

This also reverts the quick & dirty fix introduced in commit f3322c6
("flowtop: Include netlink dissector to fix build temporarily").

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>