Preserve permissions when using Pathname#atomic_write
Write install receipts atomically
Try to find install receipt when keg is unlinked

This roughly matches the logic we use to determine the active keg in
ARGV.kegs.
Use canonicalized path for loaded install receipts
Enable conceal.

Conceal is usually enabled by default.  This pull request makes conceal
enabled for homebrew.
vim: add 7.4.161 bottle.
macvim: add 7.4-72 bottle.