All link types now have a plink->precord pointer.
Add link support entry tables for const, DB and CA link types.
Route all link I/O options through the new plink->lset.
Make the *iSoftCallback devices allocate a single buffer for private
storage instead of several, and cope with calloc failures without
suspending the IOC thread.
Added new PN_LINK type in link.h, make dbStaticLib display it.
Made all device support extended so links can be changed at runtime.
Modified startup so add_record() always called before regular link processing.
Incomplete, stilll need to add MS/MSS/MSI support, currently disabled.
