[hab] Fix exec argument logic which corrupted the arg list.
[hab] Fix exec argument logic which corrupted the arg list.

This fixes several issues reported when running less trivial Supervisor
commands via `hab`, for example:

    hab start core/redis --topology leader --peer 127.0.0.1

The key point was realizing that the strings being passed into the libc
call needed to be null-terminated. There's your bit of non-safe Rust in
action, folks ;)

Signed-off-by: Fletcher Nichol <fnichol@nichol.ca>

Pull request: #572
Approved by: adamhjk

georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,nathenharvey/habitat,nathenharvey/habitat,nathenharvey/habitat,habitat-sh/habitat,nathenharvey/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat