For AIX, if dlopen() fails we try again with RTLD_MEMBER set
and a default member (shr.o or shr_64.o). However, if that
also fails, the user will receive a useless error message
that doesn't correspond to the actual problem. We now retry
the original dlopen() if the fallback to RTLD_MEMBER fails,
which has the effect of restoring the original error message.
It is possible to specify the member name in parens after the path,
e.g. sudoers.a(shr.o) for 32-bit or sudoers.a(shr_64.o) for 64-bit.
If no member is specified in the path and dlopen() fails with ENOEXEC,
try again with an explicit member, either shr.o or shr_64.o.
Previously we needed to include headers required by the various
sudo*h files. Now those files are more self-sufficient and we
should only include headers needed by code in the various .c files.
