job: Make v:job_data[2] a list.

Factor out string_to_list() from f_system()'s implementation
and use that to set job_data. This has the technical advantage of
preserving NULs, and may be more convenient for users.

Required for #1176.
job: Let vimL jobsend() accept a list.

Use save_tv_as_string(), same as vimL system(). This also makes
jobsend() more liberal in what it can accept. For example,
`jobsend(j, 123)` is now valid.

Closes #1176
job_spec: Fix bad test.