Fix compilation failures under Intel C compiler.

It appears that Intel C compiler doesn't recognize unsigned wchar_t,
however it doesn't appear to be important (the conversion function is
unused, and in other cases it doesn't appear to be needed).
Disallow Unicode conversion specifications.

This stops fish from accessing the `bool ok[UCHAR_MAX + 1]` table
beyond allocated space potentially accessing memory that doesn't
belong to fish, and crashing.
Implemented iothread_perform_on_main() to support background threads
scheduling work on main thread
Fix s/$remote// mistake.

$remote could have contained /.
Allow autosuggestions to do job expansion. Fixes
https://github.com/fish-shell/fish-shell/issues/1152
Replace autosuggestions "completions to load" mechanism with moderately
less hackish and far simpler "perform on main thread" mechanism
Merge pull request #1 from fish-shell/master

merge