unix/main: Add #if guard around -v option usage and document -i/-m opts.

This commit modifies the usage() function to only print the -v option help
text when MICROPY_DEBUG_PRINTERS is enabled.  The -v option requires this
build option to be enabled for it to have any effect.

The usage text is also modified to show the -i and -m options, and also
show that running a command, module or file are mutually exclusive.

ports/unix/main.c

@@ -305,9 +305,12 @@ STATIC int do_str(const char *str) {
 
 STATIC int usage(char **argv) {
     printf(
-"usage: %s [<opts>] [-X <implopt>] [-c <command>] [<filename>]\n"
+"usage: %s [<opts>] [-X <implopt>] [-c <command> | -m <module> | <filename>]\n"
 "Options:\n"
+"-i : enable inspection via REPL after running command/module/file\n"
+#if MICROPY_DEBUG_PRINTERS
 "-v : verbose (trace various operations); can be multiple\n"
+#endif
 "-O[N] : apply bytecode optimizations of level N\n"
 "\n"
 "Implementation specific options (-X):\n", argv[0]