iocsh command sanity checking and usage messages.

[mdavidsaver]

Naively running asynOctetConnect portname triggers a crash. Obviously not unexpected when running a new (to me) IOC shell command without first looking at help asynOctetConnect. Still not so friendly.

Maybe a codeathon project idea would be to add sanity checks (eg. when port==NULL), and/or usage messages to the IOC shell in asyn?

fyi. usage message could be printed by the iocsh call functions, and/or iocshFuncDef::usage. The later needs Base >= 7.0.4, which can also be tested with eg. #ifdef IOCSHFUNCDEF_HAS_USAGE.

https://github.com/epics-base/epics-base/wiki/Codeathon-project-ideas#add-help-for-iocsh-commands

@MarkRivers I think you should be able to edit the epics-base wiki if you want to add this, or other, project ideas.

[mdavidsaver]

For that matter, these commands could also be taught about iocshSetError() (from Base 7.0.3.1) to be able to participate in IOC shell error handle with the on command (eg. on error break)