gdb: add set/show commands for managing debuginfod

gdb: add set/show commands for managing debuginfod

Add 'set debuginfod' command.  Accepts 'on', 'off' or 'ask' as an
argument.  'on' enables debuginfod for the current session.  'off'
disables debuginfod for the current session.  'ask' will prompt
the user to either enable or disable debuginfod when the next query
is about to be performed:

    This GDB supports auto-downloading debuginfo from the following URLs:
    <URL1> <URL2> ...
    Enable debuginfod for this session? (y or [n]) y
    Debuginfod has been enabled.
    To make this setting permanent, add 'set debuginfod on' to .gdbinit.

For interactive sessions, 'ask' is the default.  For non-interactive
sessions, 'off' is the default.

Add 'show debuginfod status' command.  Displays whether debuginfod
is set to 'on', 'off' or 'ask'.

Add 'set/show debuginfod urls' commands. Accepts a string of
space-separated debuginfod server URLs to be queried.  The default
value is copied from the DEBUGINFOD_URLS environment variable.

Finally add 'set/show debuginfod verbose' commands to control whether
debuginfod-related output is displayed.  Verbose output is enabled
by default.

    (gdb) run
    Starting program: /bin/sleep 5
    Download failed: No route to host.  Continuing without debug info for /lib64/libc.so.6.

If GDB is not built with debuginfod then these commands will just display

    Support for debuginfod is not compiled into GDB.