Fix a bunch of typos.

Most are in code comments, so nothing should break. UNKOWN_COMMAND and
CMD_UNKOWN are not used elsewhere, so correcting the spelling should
also not break anything.

Fix a couple of layering violations missed in the last round.
Add missing comment headers.

Moves definitions for each layer into their own file, eliminating
layering violations in the built-in TCL code.  Updates src/Makefile.am
rules to include all files in the final startup.tcl input file, and
others Makefile.am rules to distribute the new files in our packages.