Long-standing bug: matching MTABs on pstring when mstring was intended.

This causes problems when pstring and mstring differ.  For example, when pstring is lower-case for a nice display, and mstring is UPPER CASE because, well, that's the way simh is.

strncmp won't match upcased command input to lowercase display.

The fix is trivial, but I do wonder who might be counting on the bad behavior of scp.
Merge branch 'master' of https://github.com/tlhackque/simh