vimrc: add FreeBSD style(9) plugin

Taken from FreeBSD's tools/tools/editing/freebsd.vim, (c) Sean C. Farley

The plugin registers a function, FreeBSD_Style(), which converts the
existing C/C++ code to follow FreeBSD's style(9) guideline.  By default,
it registers <Leader>-f as the function's key mapping.

In install.sh, also symlink vim/plugin inside the user's Vim directory,
unless they already have a plugin directory.
vimrc: use style(9) by default for C/C++ files

Override the augroup's defaults with the settings in plugin/freebsd.vim.
vimrc: highlight all matches of the word under the cursor

NB: May want to add highlights.vim in the future to highlight more than
one search.
zshrc: alias grep to grep --color=auto

Because looking for a pattern match in three screenfuls of text is...
somewhat inefficient.