Ticket #3232: don't owerride fish prompt.

Support for fish subshell didn't show a prompt, because fish_prompt function
was overridden to pass current path to mc.

Now user can specify a fish_prompt_mc function to use a mc specific
prompt, if none is specified fish_prompt is copied to fish_prompt_mc to
use the default prompt. The new implementation of fish_prompt calls
fish_prompt_mc to display a prompt before exit. Cwd is passed to mc as
before.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '3232_fish_prompt'

* 3232_fish_prompt:
  Ticket #3232: don't owerride fish prompt.