Previously, linking a field of e.g. a calc record to itself
(calc.INPA = calc.A) directly or through a chain of other records
that support to follow the link for attributes like units, precision
caused an infinite recursion and crashed the ioc.
Affected records: calc, calcout, seq, sub and aSub
Also added support for seq link 0 fields which had been added meanwhile.
If a sub record has an invalid INPx link, it was silently failing (and
not running the proc function). This change plumbs in the error, so
the put fails and the user knows something went wrong.
In some cases the license-identification header was missing,
so I added that as well. Replaced the remaining headers that
specifically identified "Versions 3.13.7 and higher".
Makefiles and the build system were deliberately excluded.
