Traffic Server fails to build on kfreebsd

Apache Traffic Server fails to build on Debian kFreeBSD. This is because of some special behavior when using the FreeBSD kernel with a GNU user land. In summary, the FreeBSD build stack works very well for kfreebsd, but due to some library incompatibilities, some slight modifications are required to get the build working.

I propose a patch to fix this issue, I developed it for the released 2.1.7 ATS distribution, but still succeeds on the trunk as of today with some offsests for certain hunks. Therefore you may want to merge my changes, if you want at all, rather soon to allow patch to find offsets while they aren't that significant.

The patch introduces a new host operating system "kfreebsd" which is a subset of the existing "freebsd" host definition. This is required, because the kfreebsd architecture generally seems to work with "freebsd" quirks in existing code, but still needs some special handling for some, rare, cases. Therefore, when building on kfreebsd -Dfreebsd and -Dkfreebsd is set to honor this requirements.

I briefly tested the patch for side effects on pure FreeBSD and Linux but it seems not to affect building on those systems.