Fix powerline.c styling

Also makes it compile with -std=c89 (except for snprintf) or just -std=c99,
-Wall, -pedantic.
Make it compile without warnings with -Wconversion
Do not use argparge for parsing --env and --cwd

It took 2/3 of do_render call according to profiler. New variant takes only 1/6.

WARNING: This commit will break all powerline installations with running daemon
because it changes communication protocol. You must kill and restart all your
powerline daemons for powerline to function properly.

Highlight @kovidgoyal
Merge pull request #955 from ZyX-I/faster-powerline_daemon

Do not use argparge for parsing environment