job: Fix memory leak in job_start().

If a new job cannot be started because no slots are free, we return early
without freeing the argv argument.
Mark some function arguments as [consumed] in the docs.

The argument argv of job_start() and channel_from_job() will be
freed. Mark them as such in the comments of this functions.
Try to fix problem found in the Travis Ci build.

An uv_pipe_t handle for a WStream could be left open for a
particular code path.

Patch by tarruda.
build: allow installing into the root directory (/)

It turns out that CMake always canonicalizes `CMAKE_INSTALL_PREFIX` to
an absolute path--if it's a relative path, it canonicalizes it relative
to the build directory.  As a result, the only thing the DESTDIR and
relative directory check prevents is an installation into the root
directory since CMake strips the trailing slash, turning "/" into an
empty string.  Let's just remove the check all together, since it cannot
accomplish what we intended.
Merge pull request #1429 from oni-link/fix.job_start.leak

Fix memory leak in job_start().
Merge pull request #1443 from jszakmeister/allow-root-dir-installs

build: allow installing into the root directory (/)