Change all quotes in strings and comments to the new GNU standard.

The asymmetric quotes used by GNU in the past (`...') don't render
nicely on modern systems.  We now use two \x27 characters ('...').

The proper solution would be to use the correct Unicode symmetric
quotes here.  However this has the disadvantage that the system
requires Unicode support.  We don't want that today.  If Unicode is
available a generated po file can be used to output proper quotes.  A
simple sed script like the one used for en@quote is sufficient to
change them.

The changes have been done by applying

  sed -i "s/\`\([^'\`]*\)'/'\1'/g"

to most files and fixing obvious problems by hand.  The msgid strings in
the po files were fixed with a similar command.

scd/command.c

@@ -2350,7 +2350,7 @@ update_reader_status_file (int set_card_removed_flag)
                 fname = make_filename (opt.homedir, "scd-event", NULL);
                 err = gnupg_spawn_process_detached (fname, args, envs);
                 if (err && gpg_err_code (err) != GPG_ERR_ENOENT)
-                  log_error ("failed to run event handler `%s': %s\n",
+                  log_error ("failed to run event handler '%s': %s\n",
                              fname, gpg_strerror (err));
                 xfree (fname);
                 xfree (envstr);