There was a recent question for which the answer was to use the
`-compileonly` flag but the user didn't realize that because there was
no mention on https://elv.sh/ref/command.html. Similarly, other flags,
like `-decpreation-level`, would benefit from more verbiage than shown by
`elvish -help`.
Instead of putting all possible flags in prog.Flags, flags are now registered by
the individual subprograms. The 3 flags -sock, -db and -json are shared by
multiple subprograms and still handled by the prog package.
This new design allows separating the support for -cpuprofile into a separate
subprogram, which is no longer included by the default entry point, making the
binary slightly smaller. A new entrypoint "withpprof" is created.
Also include the LSP subprogram in the nodaemon entry point.
