GitLab

This is because if the user specifies that they want
to mount multiple times, something must be wrong. try
unmounting to reset things and then proceed.

This commit adds a Mount abstraction (which is really just
a wrapped context closer). It makes sure to bind the mount
to the fate of the Node (i.e. close it if the node ends).
This fixes #350

Also:
- map[cmds.EncodingType]cmds.Marshaller -> MarshalMap

cc @mappum @maybebtc

@whyrusleeping @jbenet

is this non-deterministic?

fix(mount_unix) rm extraneous error

@mappum can you just sanity check me here with a quick LGTM CR?

@mappum, could you provide a quick LGTM?