Replace autosuggestions "completions to load" mechanism with moderately
less hackish and far simpler "perform on main thread" mechanism
autoconf build: clean up Makefile, remove fish.spec

Removes some unused variables and out-of-date references.
Wraps some tests in quotes to avoid expansion errors.
Removes the fish.spec generated file as it is out of date and is
arguably better maintained by downstream packagers.
See http://github.com/zanchey/fish-build/ for a better RPM spec file.
add correct version number to manpage output
Makefile.in: remove cruft
__fish_git_prompt: initialise colors early

Otherwise __fish_git_prompt_informative_status tries to expand an unset
variable.

Closes #1157.