Support shared (read-only) locks in addition to exclusive locks

[bielr]

It would be nice if this crate supported read/write locks, which as far as I know, could be easily implemented using libc::LOCK_SH on Unix and removing the minwinbase::LOCKFILE_EXCLUSIVE_LOCK flag on Windows.

The API for this either means a separate SharedLockFile type or changing the type of the LockFile::locked field to an enum (so that the type of locking used can be queried). The former is probably cleaner and can easily be feature-gated.