MacRelixLib:  Place member definitions together
Genie:  Remove unused OSErrno.hh include
relix:  Factor out relix-api package

Code in relix-kernel can't link-depend on code in MacRelixLib as long as
the former is a prerequisite of the latter.  So split out the api/ parts
into their own package.
relix:  Add errno_from_os_exception()
relix, Genie:  Add relix::set_errno()
relix:  Add set_errno_from_exception()
Genie:  Use relix::set_errno()
Merge branch 'set_errno'
Genie:  Remove OSErrno prerequisite
MacRelixLib:  Silence bogus missing-return warning