If the underneath file system is a Network File System,
`brew cleanup` will fail to remove the lock files with following error
message:
Error: Bad file descriptor @ rb_file_flock - /path/to/the/lock_file
This commit fixes such issue and adds the corresponding test case.
Fixes#2355
Create unremovable_kegs instance var
Check cellar cleanup failure after full cleanup completes
Use module_function in Homebrew::Cleanup as we never instantiate the
class