gdb: rework "set debuginfod" commands

gdb: rework "set debuginfod" commands

As discussed here [1], do some re-work in the "set debuginfod commands".

First, use "set debuginfod enabled on/off/ask" instead of "set
debuginfod on/off/ask".  This is more MI-friendly, and it gives an
output that makes more sense in "info set", for example.

Then, make the show commands not call "error" when debuginfod support is
not compiled in.  This makes the commands "show" and "show debuginfod"
stop early, breaking gdb.base/default.exp:

    Running /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.base/default.exp ...
    FAIL: gdb.base/default.exp: info set
    FAIL: gdb.base/default.exp: show

 - Make the "debuginfod enabled" setting default to "off" when debuginfod
   support is not compiled in, and "ask" otherwise.
 - Make the setter of "debuginfod enabled" error out when debuginfod
   support is not compiled in, so that "debuginfod enabled" will always
   remain "off" in that case.
 - Make the setter of "debuginfod verbose" work in any case.  I don't
   see the harm in letting the user change that setting, since the user will
   hit an error if they try to enable the use of debuginfod.
 - I would do the same for the "debuginfod urls" setter, but because
   this one needs to see the DEBUGINFOD_URLS_ENV_VAR macro, provided by
   libdebuginfod, I made that one error out as well if debuginfod
   support is not compiled it (otherwise, I would have left it like
   "debuginfod verbose".  Alternatively, we could hard-code
   "DEBUGINFOD_URLS" in the code (in fact, it was prior to this patch,
   but I think it was an oversight, as other spots use
   DEBUGINFOD_URLS_ENV_VAR), or use a dummy string to store the setting,
   but I don't really see the value in that.

Rename debuginfod_enable to debuginfod_enabled, just so it matches the
setting name.

[1] https://sourceware.org/pipermail/gdb-patches/2021-October/182937.html

Change-Id: I45fdb2993f668226a5639228951362b7800f09d5
Co-Authored-By: Aaron Merey <amerey@redhat.com>