Remove a.out defines in OpenBSD and NetBSD mkspec

OpenBSD made the switch from a.out to ELF with release 3.4, published
2003, see http://www.openbsd.org/34.html. In preparation to cleaning
up the mkspecs for OpenBSD, remove the a.out defines as only recent
versions of OpenBSD (5.8 onward) will be supported.

NetBSD switched from a.out to ELF file format with release 1.5, see
https://www.netbsd.org/releases/formal-1.5/NetBSD-1.5.html

 in the year
2000.

Remove the defines for older a.out systems now as we are at release 7.0
and only 6.0 onwards releases are currently supported anyway.

While cleaning up, remove the old comments for QT_SOCKLEN_T values of
outdated OpenBSD and NetBSD releases as well.

Change-Id: I8519eab7bcd4af19c0e9f628657878f32b2bd602
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>