Better auto-completion in psci.

* completion for the names of the interactive commands (this makes names
  longer that one character more viable);
* context-aware completion of module names, identifiers, and filesystem
  paths depending on command;
* trying to complete an empty prompt shows the list of all identifiers
  and `:`-commands.
Merge pull request #789 from vkorablin/master

Better auto-completion in psci.