lib{c,sys}: stop exposing errno symbol

[brooks]

Summary

Officially since C11 (and in reality FreeBSD since 3.0 with commit
1b46cb523df3) errno has been defined to be a macro. Rename it to
__errno and move it to FBSDprivate_1.0 for use by libthr. Add a
FBSD_1.0 compat symbol for existing binaries that were incorrectly
linked to the errno symbol during libc.so.7's lifetime.

This breaks linking previously broken software that directly linked to
errno.

[kib]

Does this solve the ABI issue that we have two errno's for binaries that did 'extern int errno;' explicitly? I suspect not, since they should get a COPY reloc resolved from libc errno, and now it would either error out on image activation, or still be silently broken.

I believe an experiment is due.

lib/libsys/Symbol.sys.map

386

Can we rename it even more, to make it less confusing? E.g. __libsys_errno.