Fix sysconf errno for unknown names.

The code (and comment) have been like this since the initial commit, but
there was never a test of this, and glibc seems to have been returning
EINVAL (as POSIX says it should) since long before Android existed.

Bug: http://b/31154352
Test: ran tests
Change-Id: Ica1ea836a4a5699ef0d956fe9c0f322e567de34d