If HOMEBREW_KEEP_INFO environment variable is set:
- Do not symlink the info directory file (aka 'share/info/dir')
otherwise it gets overwritten by next installed brew.
- Install an entry in the directory for each linked info file when the
brew is linked.
- Uninstall the entry when the brew is unlinked.
ClosesHomebrew/homebrew#9700.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>