main.cc: Show '-d=no-foo' in help screen (#2814)
This commit makes the output of 'lilypond --help' look like this:
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-d, --define-default=SYM[=VAL] set Scheme option SYM to VAL (default: '#t')
-d, --define-default=no-SYM set Scheme option SYM to '#f'
-d, --define-default=help show help for Scheme options
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Closes #2814 (closed).