Do not treat first element as argument in line_pos

It's command name.
Some style changes for commands_completion.c
Skip escaping for out of arg on command-line
Some style updates for engine/options.c
Separate escaped and raw matches in compl iface
Add more tests for command-line escaping

Cover cases not covered before and fixed in the previous commit.
Merge branch 'fix-too-much-escaping-on-command-line'

Fixed excess escaping added after introducing completion hook.