Add proper dockerinit path support for distros that are more strict about the FHS

So, it turns out that even though many distros use "/usr/libexec/docker" as the proper place for us to put binaries that aren't supposed to be user-executable, the FHS (2.3) specifies that they're supposed to be in "/usr/lib/docker" instead (so, platforms like Arch Linux need this).

/cc @seblu