Don't return NULL when we are freeing the root node; return a free'd pointer
instead.

PR/50681: Markiyan Kushnir: Fix memory leak when we delete the root node.
It is questionable if we should return NULL in that case, but what is the
parent of root? The new adjusted root?

Update old-style definitions to ANSI, remove a couple of register
definitions along the way. Fixed gcc 4.1 build (thank you vax)

branches: 1.5.4;
Remove __P()

Coverity CID 2528: Don't delete the node we are about to return.

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

t{search,find,walk,delete} from OpenBSD via Neil A. Carson