KGDB: Allow modules to be loaded from the specified kernel's directory.

When looking up the absolute path for a kld, call find_kld_path() first.
This enables locating the module in a different directory than the one
stored in kernel memory.

With this change, kgdb can now be run on a kernel & vmcore whose associated
modules are located in the same directory as the kernel. This makes
independent triaging of problems much easier.

This change also does not break the normal kgdb use case where no arguments
are specified; in that case kgdb loads the running kernel and its modules.

Reviewed by: adrian
Approved by: ken (mentor)
Sponsored by: Spectra Logic
MFC after: 1 month