Netif.attach (UNIX backend) is now working on Linux

I’m using a whitespace killer so here is a summary of changes:

* netif.ml : added a comment
* netif.mli : added a comment
* _tags: added the annot flag because I like being able to see types with tuareg
*  tap_stubs_linux.c : implemented pcap_opendev. Actually this is totally unrelated to pcap, but this is the function used to open an existing tap interface on Netif, so the code in pcap_opendev will fulfill that role. We should clarify the name eventually.

Fixed tap_opendev (was missing CAMLparam/CAMLreturn)

Basically, under Linux, the same code can be used to create a tap interface, or to open an existing one. Unlike in MacOSX, no trick is needed.

This should justify why the code in pcap_opendev actually do what it should.
But there is no pcap involved whatsoever in Linux.