brew-info: Add pinned status to info.
Closes mxcl/homebrew#18851.
brew-pin: Fix bug in `FormulaPin#pinned?`
Since `pinned?` was using `File#exists?`, broken symlinks left by
force-upgraded formulae made `pinned?` return false while `pin` returned
with an error because it could not overwrite the broken link. This is
fixed by using `File#symlink?` instead, which returns true for broken
symlinks and false only when they do not exist.