fix rewinddir on nfs. fix PR/42879 (and probably PR/40229.)

PR/24324: Arne H Juul: Re-implement seekdir/telldir using a pointer of
locations per directory instead of a global hash table to avoid memory
leak issues, and incorrect results.

rename __func to _func_unlocked, and add their prototypes in extern.h
instead of exposing them in dirent.h. More locking consistency fixes.

Move UCB-licensed code from 4-clause to 3-clause licence.

Patches provided by Joel Baker in PR 22280, verified by myself.

Delint.
Remove trailing ; from uses of __weak_alias(). The macro inserts this if
needed.

back out the #ifdef _DIAGNOSTIC argument checks; too many people complained.
_DIAGASSERT() is still retained.

* use _DIAGASSERT() to check pointer arguments against NULL and file
descriptors against -1 (as appropriate).
* add actual checks which to detect stuff that would trigger_DIAGASSERT(),
and attempt to return a sane error condition.
* knf some code
* remove some `register' decls.

the first two items result in the addition of code similar to the
following in various functions:

_DIAGASSERT(path != NULL)
#ifdef _DIAGNOSTIC
if (path == NULL) {
errno = EFAULT;
return (-1);
}
#endif

If port provides __weak_alias(), provide an Standard C and POSIX pure
identifier namespace by renaming non standard functions and variables
such that they have a leading underscore. The library will use those
names internally. Weak aliases are used to provide the original names
to the API.

This is only the first part of this change. It is most of the functions
which are implemented in C for all NetBSD ports. Subsequent changes are
to add the same support to the remaining C files, to assembly files, and
to the automagically generated assembly source used for system calls.
When all of the above is done, ports with weak alias support should add
a definition for __weak_alias to <sys/cdefs.h>.

Fix RCSID's

branches: 1.4.4;
clean up Id's on files previously imported...

branches: 1.3.2; 1.3.4;
Add RCS Id's

Return NULL and set errno to ENOTDIR if opening a non-directory (POSIX.1,
Section 5.1.2.4). Rename _seekdir to __seekdir, as former is in the
user's namespace.

branches: 1.1.1;
Initial revision