Don't abort on an unimplemented sysconf parameter. One of the documented
functions of sysconf(3) is to "test ... whether certain options are
supported", so programs are free to test for features we don't support
yet, and we're supposed to return -1 with errno set to EINVAL.

For example, Boost tested _SC_THREAD_SAFE_FUNCTIONS which we didn't
support. It would have been fine if we set EINVAL (it would switch
from *_r functions to the non-reenatrant ones) - but it wasn't fine
that we abort()ed because of this test :-)

To be on the safe side, this patch still prints a message if we see an
unknown sysconf - in case in the future we'll come across a new one we
must treat. But eventually, the message should go away too.