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
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.
