Add my_memrchr() implementation of memrchr()

Add my_memrchr() implementation of memrchr()

On platforms that have memrchr(), my_mrchr() maps to use that instead.

This is useful functionality, lacking on many platforms.  This commit
also uses the new function in two places in the core where the comments
previously indicated it would be advantageous to use it if we had it.

It is left usable only in core, so that if this turns out to have been a
bad idea, it can be easily removed.