Add support for 16-bit verbnums in I6 library w/ Glulx

Created by: jessepav

Compiler 6.32 added support for 16-bit verbnums when using Glulx, but the library never took advantage of them. I've backported a few changes from I7 to support this, and they seem to work, though more knowledgeable eyes may want to look the changes over.

Edit: after looking through the compiler sources in verbs.c and text.c, I'm more confident that these patches should be okay.