Use the UAUTO SYSCTL type for exporting the bounce zone alignment,
because the variable size depends on the build type.

Reported by:	kib @
MFC after:	3 days
Mark some more sysctl stuff shared-locked and MPSAFE.
older binutils does not know about --no-fatal-warnings
Revert r273426 r273409

A solution that work with both new and old binutils should be investigated