[Ada] General improvements in ada-tasks.c

[Joel Brobecker <brobecker at adacore dot com>]

Hello,

This series of patches implements two things that I have wanted to
do for quite a while:
  - Remove some of the static globals that we were using in that
    module, to make them per program space, or per inferior;
  - Use ui-out instead of printf to print the list of Ada tasks.

The latter is necessary work for implementing the GDB/MI equivalent
of the `info tasks' command, but I will send that as a separate
thread.

All patches in this series have been tested on x86_64-linux,
and checked in.