vim: snippet tweaks

- Add GPLv2 and GPLv3 snippets for C.

- Fix use of C snippets in C++.
vim: do not auto-wrap plain text by default
vim: strip trailing whitespace on save

This change is made in vimrc_local's rather than in the global .vimrc
because a lot of open-source projects have trailing whitespace, and
stripping trailing whitespace on save in these projects results in noisy
patches.
vim: change gvim to 117x60

With Anonymous Pro 11 in Xubuntu's default panel setup, this size almost
exactly fits half the screen.
vim: move vimrc_g3 to local/special/

As vimrc_g3 isn't really a vimrc_local (as it includes .vimrc rather
than the other way around), don't put it in local/.