Code review comment for ~bhill/epics-base:pvAccess-envDefs.h

I was reviewing this merge request to see what remaining work I needed to do and am not convinced any more changes are needed other than perhaps modifying pvAccessCPP to allow EPICS_PVA* defaults to be configured via configure/CONFIG_ENV.

As I see it, there are two issues here.
1. Allowing configuration of CA and PVA defaults via configure/CONFIG_ENV. This isn't something that we do at SLAC, but should perhaps be supported for PVA for sites which use CONFIG_ENV. Since most sites likely follow our practice and configure via scripts that set the actual environment variables, the values in CONFIG_ENV are only knowable if you examine the file or startup a virgin softIOC and use envGet*() calls to see the default values.

2. Allowing runtime examination of CA and PVA settings via the various envGet*() functions and modules such as iocStats that expose them as PVs.

I think the current state of the patch supports the 2nd use case, and a separate patch would be needed for pvAccessCPP to support build time configuration of PVA defaults.