Port to Neovim: Add compatibility layer for job control and output buffering

Add Neovim-compatible wrappers for job control (s:JobStop, s:ChanSend,
s:ChanCloseIn, s:ChanSetCallback, s:JobInfo, s:GetJobBufNr), emulate
Vim's 'out_io':'buffer' behavior via s:NvimOut/s:NvimExit callbacks,
handle signal sending via vim.loop.kill, initialize script-local click
variables, fix readdir() call (two parameters, not three), and fix
various Vimscript and callback regressions introduced during porting.

Author: mcepl