Deprecating userland a.out support

FreeBSD has used ELF binaries/libraries for decades, but still has
support for legacy a.out binaries. Portions of this have been retired
over time, and I propose removing the remaining pieces before FreeBSD
13. I'm not proposing making any change to kernel a.out support, but
users needing userland a.out tools would have to install them from old
FreeBSD releases.

I have opened the following reviews for three tools that still support a.out:

D27478 ldd: Retire a.out support
D27480 gprof: Retire a.out support
D27481 ldconfig: Retire a.out support

If there are no objections I plan to commit these changes before the
end of the month, along with small changes to the mtree file (removing
aout directories) etc.