Unify parse_argv style

getopt is usually good at printing out a nice error message when
commandline options are invalid. It distinguishes between an unknown
option and a known option with a missing arg. It is better to let it
do its job and not use opterr=0 unless we actually want to suppress
messages. So remove opterr=0 in the few places where it wasn't really
useful.

When an error in options is encountered, we should not print a lengthy
help() and overwhelm the user, when we know precisely what is wrong
with the commandline. In addition, since help() prints to stdout, it
should not be used except when requested with -h or --help.

Also, simplify things here and there.

src/socket-proxy/socket-proxyd.c

@@ -589,17 +589,13 @@ static int add_listen_socket(Context *context, int fd) {
         return 0;
 }
 
-static int help(void) {
-
-        printf("%s [HOST:PORT]\n"
-               "%s [SOCKET]\n\n"
+static void help(void) {
+        printf("%1$s [HOST:PORT]\n"
+               "%1$s [SOCKET]\n\n"
                "Bidirectionally proxy local sockets to another (possibly remote) socket.\n\n"
                "  -h --help              Show this help\n"
                "     --version           Show package version\n",
-               program_invocation_short_name,
                program_invocation_short_name);
-
-        return 0;
 }
 
 static int parse_argv(int argc, char *argv[]) {
@@ -620,12 +616,13 @@ static int parse_argv(int argc, char *argv[]) {
         assert(argc >= 0);
         assert(argv);
 
-        while ((c = getopt_long(argc, argv, "h", options, NULL)) >= 0) {
+        while ((c = getopt_long(argc, argv, "h", options, NULL)) >= 0)
 
                 switch (c) {
 
                 case 'h':
-                        return help();
+                        help();
+                        return 0;
 
                 case ARG_VERSION:
                         puts(PACKAGE_STRING);
@@ -638,7 +635,6 @@ static int parse_argv(int argc, char *argv[]) {
                 default:
                         assert_not_reached("Unhandled option");
                 }
-        }
 
         if (optind >= argc) {
                 log_error("Not enough parameters.");