Add memBEGINs(), memENDs() and use them in APItest

Add memBEGINs(), memENDs() and use them in APItest

memBEGINs() is like strBEGINs(), but can be used for buffers without
trailing NULs.  It can also be used when there is a trailing NUL, and
the length is known, as it should be somewhat faster, only having to
check for one ending condition.

Same for memENDs vs strENDs