This is a non-portable header, and we cannot expect it to be provided by
the system libc (e.g. musl). We just need and rely on declaration that
we have defined ourselves in our own <bsd/sys/cdefs.h>. So we switch to
only ever assume that.
Fixes: https://bugs.freedesktop.org/105281
Some libc libraries do not have an <a.out.h> header. And a.out as an
executable format is very much obsolete on pretty much all currently
supported systems, even if they might still support loading such
objects.
Remove the a.out support to increase portability.
First stage of the transition to avoid possible clashes with other
software by moving out of the way the remaining headers from
/usr/include/.
At least nlist.h is known to cause file conflicts with some libelf
implementations. libutil.h is not really complete and might cause
confusion if software detects its availability w/o someone actually
checking. And lastly vis.h is not known to cause any problem and it's
complete, but better be safe than sorry.
The compatibility headers will be removed in a later release.
