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.
wm4
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