FreeBSD_HEAD - Build #3352 - Fixed

FreeBSD_HEAD - Build #3352 - Fixed:

Build information: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD/3352/
Full change log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD/3352/changes
Full build log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD/3352/console

Change summaries:

288915 by hrs:
Reallocate a maxlen-long buffer only when the current maxlen is
shorter than the required length.  Note that it rarely happens
because maxlen is almost always 128 which covers struct sockaddr_storage.

288914 by hiren:
Add a comment specifying how we implement rfc3042.

Differential Revision:	D3746
MFC after:	    1 week
Sponsored by:	    Limelight Networks

288913 by ngie:
Use LIBXOSRC instead of LIBXO when defining the path to contrib/libxo

The latter is already defined in bsd.libnames.mk, so avoid the conflict
in case someone copy-pastes make variables

While here, switch path to the top of the source tree with SRCTOP