fix tsearch, tfind, tdelete to handle null pointer input

POSIX specifies the behaviour for null rootp input, but it
was not implemented correctly.

tsearch code cleanup

changed the insertion method to simplify the recursion logic and
reduce code size a bit.

fix tsearch to avoid crash on oom

malloc failure was not properly propagated in the insertion method
which led to null pointer dereference.

fix tdelete to properly balance the tree

the tsearch data structure is an avl tree, but it did not implement
the deletion operation correctly so the tree could become unbalanced.

reported by Ed Schouten.

fix aliasing violations in tsearch functions

patch by nsz. the actual object the caller has storing the tree root
has type void *, so accessing it as struct node * is not valid.
instead, simply access the value, move it to a temporary of the
appropriate type and work from there, then move the result back.

search: add comments to tsearch_avl.c

XSI search.h API implementation by Szabolcs Nagy