Allow fscrypt to work in containers (#213)

Allow fscrypt to work in containers (#213)

Update the /proc/self/mountinfo parsing code to allow selecting a Mount
with Subtree != "/", i.e. a Mount not of the full filesystem.  This is
needed to allow fscrypt to work in containers, where the root of the
filesystem may not be mounted.

See findMainMount() for details about the algorithm used.

Resolves https://github.com/google/fscrypt/issues/211