brew-pin: prevent selected formulae from upgrade.

* Added `pin` et. al. to manpage.
* Added `brew pin` to `brew.1` * Added `brew unpin` to `brew.1`
* Added `brew list --pinned` to `brew.1`
* Added information about frozen formulae to `brew upgrade` in `brew.1`
* Added `pin` et.al. to completion scripts.
* Unpin formulae when uninstalling them
* Unpin and re-pin formulae when upgrading (avoids stale symlink)

References #18386.
Closes #18515.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>