GCC8 now spots and warns about such mismatches.
And we should take such warnings seriously;
code produced by the newer GCC versions tends to segfault,
especially under -O2 and above, when a return statement is
actually missing, even if the return value is actually not
used at call site.
Here, a functor to unlock the active "guard" is passed into
a macro construct, which basically allows to abstract the
various kinds of "guards", be it mutex, condition variable
or the like.
Seemingly, the intention was to deal with a failure when
unlocking -- however all the real implementations prefer
to kill the whole application without much ado.
* add a 'unknown' error to the error system as fallback
* lockerror.c|h define all errors which can happen due locking
* lumiera_lockerror_set() translates posix errors to lumiera errors
* remove stale errors from sectionlock.h
