conf, passt: Rename stderr to force_stderr

While building against musl, gcc informs us that 'stderr' is a
protected keyword. This probably comes from a #define stderr (stderr)
in musl's stdio.h, to avoid a clash with extern FILE *const stderr,
but I didn't really track it down. Just rename it to force_stderr, it
makes more sense.

[sbrivio: Added commit message]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

passt.h

@@ -158,7 +158,7 @@ struct ip6_ctx {
  * @trace:		Enable tracing (extra debug) mode
  * @quiet:		Don't print informational messages
  * @foreground:		Run in foreground, don't log to stderr by default
- * @stderr:		Force logging to stderr
+ * @force_stderr:	Force logging to stderr
  * @nofile:		Maximum number of open files (ulimit -n)
  * @sock_path:		Path for UNIX domain socket
  * @pcap:		Path for packet capture file
@@ -207,7 +207,7 @@ struct ctx {
 	int trace;
 	int quiet;
 	int foreground;
-	int stderr;
+	int force_stderr;
 	int nofile;
 	char sock_path[UNIX_PATH_MAX];
 	char pcap[PATH_MAX];