Mac OS X:

Bypass the dlfcn compatibility layer in lookupSymbol, use Apple's
NSModule functions directly instead. This protects us from differences
between different implementations of the dlcompat library (regarding
treatment of prefixed underscores).
Note that the compatibility layer is still used in addDLL (it's needed by
the POSIX library anyway).

Also, add a little more sanity checking (checkProddableBlock) to the Mach-O
linker itself.

MERGE TO STABLE