Cygwin: set ENOSHARE and ECASECLASH _sys_errlist[] entries to empty

Cygwin: set ENOSHARE and ECASECLASH _sys_errlist[] entries to empty

These errno values are no longer used by Cygwin.  Change the entries
to empty strings instead of NULL to avoid crashes in existing
binaries directly accessing the table.  Enhance strerror_worker()
such that empty strings also result in "Unknown error ..." messages.
Also add a static_assert check for the _sys_errlist[] size.

Signed-off-by: Christian Franke <christian.franke@t-online.de>