quill(n) os(n) module

This is a grab bag.

* Defined new quill(n) module, os(n), with test and man page.  It
contains specific OS-independence calls from quillapp/plat.tcl
* Defined new tool, 'quill replace', to do global replaces across many
files, because I find this convenient.
* Defined env.tcl as a wrapper around plat.tcl, and updated the rest of
the app to use it.

env.tcl might replace plat.tcl all together; it's not clear yet.