Add support for 'docker exec'

This patch brings in support for running a new command in an existing and active container.
Fixes #6777 and #1228.

Some use cases:
* Perform administrative tasks - add/remove devices, update files, etc
* Debugging a running container - gdb, ssh
* Run periodic commands
* Run commands that are not part of the static container specification.

API
Client: 'docker exec redis touch /tmp/file'
Remote: '/containers/redis/exec'

Well, `strcmp` isn't vulnerable unless you're doing cryptographic comparisons (where you need to use strictly `O(n)` comparisons for any inputs). There's no danger of buffer overflow (since there's no writes occuring) or any real out-of-bounds errors (since IIRC strcmp compares lengths to give you a positive or negative return value).