tool_listhelp: easier to generate with gen.pl

tool_listhelp.c is now a separate file with only the command line --help
output, exactly as generated by gen.pl. This makes it easier to gen

tool_listhelp: easier to generate with gen.pl

tool_listhelp.c is now a separate file with only the command line --help
output, exactly as generated by gen.pl. This makes it easier to generate
updates according to what's in the docs/cmdline-opts docs.

cd $srcroot/docs/cmdline-opts
./gen.pl listhelp *.d > $srcroot/src/tool_listhelp.c

With a configure build, this also works:

make -C src listhelp

Closes #7787

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