These definitions are scattered throughout the codebase which makes it
hard to refactor them later (my goal is to move them outside of
HOMEBREW_LIBRARY). Unify their definitions for clearer code and easier
movement later.
Since #292, HOMEBREW_CACHE was moved to a per-user directory. This makes
it unsuitable to store global lock files on multiple users environment.
Therefore, introducing a global lock directory `/Library/Lock.d` to
store lock files from formula lockers as well as `brew update`.
The formula locks used by the installer and commands like link and
unlink are backed by open files and flock(). The open file descriptors
are thus leaked to any subprocesses. This can result in weird behavior
in programs spawned from formula that do not expect to inherit these
descriptors.
Fix this by setting close-on-exec on the lock file descriptors.
FixesHomebrew/homebrew#21486.
