Start jobs without stdin being open

[blueyed]

Currently there is no way to start a job with closed stdin, which might result in tools like ripgrep to falsely assume that stdin is being used (BurntSushi/ripgrep#1892).

chanclose(jobid, 'stdin') can be used to close it, but only after jobstart, so that usually the detection of stdin happened already by then then.

I have not checked if the ChanOpen autocommand could be used for this maybe, in case it is blocking - but it would certainly be easier to allow for specifying this with a job's options already.
When doing this it might also make sense to add support for closing (or redirecting maybe) stdout/stderr then, but the latter is a bigger topic (#8823).

ref: mhinz/vim-grepper#162

[BurntSushi]

I believe this PR is trying to do just this: #14812