BSD mkspec cleanup for FreeBSD, NetBSD and OpenBSD to common namespace

BSD OS mkspecs share mainly the same configurations except very few
differences. Merge into a common/bsd/bsd.conf file to be used across all BSD
OSes in their respective qmake.conf and add a qplatformdefs.h that contain
the common defines to be re-used in the BSD mkspecs.

The change includes the usage of <sys/param.h> also on NetBSD through the
common qplatformdefs.h, which is intended for using NetBSD's version defines.

Change-Id: Ibb0ac9e4c8bb5aff7d0febdcab1a4b9600a61117
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>