Reenable interactive operator completions

[gracjan]

Disabled in #1247.

GHCi prior to version 8.0.1 have bug in :complete command: when completing operators it returns a list of all imported identifiers (see Track ticket). This leads to significant Emacs slowdown. To aviod slowdown operator completions are disabled for now.

Suggested solution:

1. In the beginning of a session issues a test :complete >>= command.
2. See if output is okay or not okay.
3. Mark GHCi session as one with a working :complete or a broken :complete command.
4. Do not complete operators in sessions with broken complete.