cmd: do not use a random value for MP_CMD_OPT_ARG

This flag is used only by the command parser. Its value overlapped with
some of the existing m_option flags, but only flags that did not matter
for the command parser (i.e. the flag bits used had mostly private uses
in each component). It's still a bit unclean and dangerous to use an
essentially random value, so reuse M_OPT_OPTIONAL_PARAM for it.

Since M_OPT_OPTIONAL_PARAM has a slightly longer name than
MP_CMD_OPT_ARG, I'm going to keep the old name.

input/cmd.h

@@ -24,7 +24,7 @@
 #include "options/m_option.h"
 
 #define MP_CMD_DEF_MAX_ARGS 9
-#define MP_CMD_OPT_ARG 0x1000
+#define MP_CMD_OPT_ARG M_OPT_OPTIONAL_PARAM
 
 struct mp_log;
 struct mp_cmd;