Due to some very old mistakes too many of Delve's flags are declared as
persistent on cobra's root command. For example the headless flag is a
global flag but does not apply to connect, dap or trace; the backend
flag does not apply to replay, core and dap; etc.
Almost all global flags should have been declared as local flags on
individual subcommands. Unfortunately we can not change this without
breaking backwards compatibility, for example:
dlv --headless debug
would not parse if headless was a flag of debug instead of a global
flag.
Instead we alter usage function and the markdown generation script to
strategically hide the flags that don't apply.
Fixes#2361
