Vim: Do not use tw=72 by default

I often find myself typing ":set tw=0". Let's try turning it off.

I DO want tw=72 for some filetypes, though. And I DO want to have a
default textwidth of 72 for re-wrapping.

Note that the "gitcommit" ftplugin sets tw=72 by itself and so does
"mail".
Vim: Add a convenience function to toggle tw=(0|72)
Mutt: Don't use "mailgroff" anymore

People complained too much about "unreadable" mail because their client
fucked things up...
Revert "Vim: Do not use tw=72 by default"

This reverts commit 9780e24977f05c0b2525c747c52b3f0b73262eaa.

Blech, too many ftplugins set the textwidth to some weird value. I'll
keep the ",W" hotkey and that's it.
Vim: Use glob() for .vimrc.local
Vim: ToggleTextwidth: Use setl