Fixed shared library unloads when the unloaded library doesn't come off

the end of the list. We had an issue in the MacOSX dynamic loader where if
we had shlibs:
1 - a.out
2 - a.dylib
3 - b.dylib

And then a.dylib got unloaded, we would unload b.dylib due to the assumption
that only shared libraries could come off the end of the list. We now properly
search and find which ones get loaded.

Added a new internal logging category for the "lldb" log channel named "dyld".
This should allow all dynamic loaders to use this as a generic log channel so
we can track shared library loads and unloads in the logs without having to 
have each plug-in make up its own logging channel.

llvm-svn: 118147