Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r239151: ftw(): Do not check the maxfds argument against OPEN_MAX.

Apart from the fact that nothing should have OPEN_MAX as a limit (as opposed
to RLIMIT_NOFILE from getrlimit() or _SC_OPEN_MAX from sysconf()), POSIX
does not require us to check this. POSIX does have a requirement on the
application that maxfds not exceed {OPEN_MAX}, but does not require the
implementation to check it ("may fail").

PR: 95239

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Sync with OpenBSD (zap rcsid).

MFC after: 2 weeks

Replace the current implementations of ftw() and nftw() with the OpenBSD
implementations written by Todd C. Miller. These are cleaner, less buggy
and actively maintained.

Don't pass function pointers via a void * parameter.

Remove unused variable.

Noticed by: Todd Miller <Todd.Miller@courtesan.com>

Add implementations of ftw(3) and nftw(3) and the corresponding header
ftw.h. This is the implementation written by Joel Baker
<fenton@debian.org> for inclusion in NetBSD, but with several
bugfixes.

Obtained from: Debian