Merge all instances of BSD headers into one place (WIP/Help requested)

[TwoFx]

BSD library headers exist in several places, most notably /headers/compatibility/bsd/ and /src/libs/compat/freebsd_network/compat. The latter is a significantly more complete version, but it is unavailable to normal apps outside the tree. Making these headers available is crucial for being able to port applications which natively run on BSD to Haiku.

I have attached a WIP of merging the sys/ and machine/ folders of compatibility and freebsd_network to compatibility, where they would be available to all apps which want to use them.

As you can see, this change is quite involved. In particular, the freebsd_network headers will depend on the compatibility/bsd headers, which means that all instances in the tree that use the freebsd_network headers need to have their Jamfile modified. I did that for a handful of instances where this comes up, but before I continued doing this change, I wanted to ask for comments whether this change is desirable in general.