[RFAv3 0/3] Convenience functions $_gdb_setting/$_gdb_int_setting

[Philippe Waroquiers <philippe dot waroquiers at skynet dot be>]

As part of the discussion of 'show | set may-call-functions [on|off]',
Eli suggested to have a way to access the GDB settings from
user defined commands.

So, this patch series implements this.

2 functions are provided:
  * $_gdb_setting that gives access to all settings, giving back a string value.
  * $_gdb_int_setting, giving access to boolean, auto-boolean and integers.
    This is easier to use for such types than the string version.

This is v3.
Compared to v1, it rebases to a recent master, and handles the comments
of Eli about the documentation.

(there was in fact no changes between v2 and v1, because I forgot to
commit the changes before sending the RFAv2 mail).