libdwfl: dwfl_standard_argp should not fail when not able to attach Dwfl.

libdwfl: dwfl_standard_argp should not fail when not able to attach Dwfl.

As pointed out in https://bugzilla.redhat.com/show_bug.cgi?id=1107654
commit 191080 introduced a thinko that caused dwfl_standard_argp
to fail if the Dwfl couldn't be attached. Instead of generating a warning
as the comment intended, the failure would be fatal. But even warning
about dwfl_core_file_attach () or dwfl_linux_proc_attach () failing
would be a mistake. The caller/user might not be interested in such
a non-fatal issue. So just ignore if the call failed for whatever reason.
If the caller is interested in warning up front about this issue, then
dwfl_pid () should be called to check the Dwfl is attached. Things should
work just fine for anything that doesn't call any of the dwfl_state related
functions.

Signed-off-by: Mark Wielaard <mjw@redhat.com>