Add LibPath for shared library initialization

[bpbecker]

Currently you need to put the shared libraries in one of a few specific places, some of which require administrator access. We should add an optional LibPath variable to the Math namespace. If specified, this is where Math will look first for the shared libraries.

Ideally I'd like to see the following search hierarchy implemented:

• LibPath, if non-empty
• the directory with the dyalog executable
• the current directory
• the Windows\System directory (Windows only)
• the Windows\ directory (Windows only)
• a directory in the operating system's PATH
• the directory where Math.dyalog was loaded from (if available)
• the directory of the current workspace (if applicable)