realpath.c

The former implementation didn't necessarily provide all versions of
realpath() that might be invoked, and didn't necessarily invoke the
proper version.  That led to possible crashes and possible unnecessary
semantic differences.  This fixes all that, though it doesn't retrofit
the 10.5+ semantic changes to 10.4 (and neither did the earlier
implementation).

The former implementation also failed to return any internally
allocated buffer on failures, causing a memory leak.  That is now
fixed as well.

TESTED:
Builds and passes all tests (including expanded tests) on all
platforms, with all applicable SDKs.