Split ParseBindPath from config Get/Setters & add basic tests, from sylabs 319

[edytuk]

This pulls in sylabs PR

• Split ParseBindPath from config Get/Setters & add basic tests sylabs/singularity#319

The original PR description was:

The parsing routines for bind mount specifications are buried among a large amount of simple Getter/Setter code for the main config struct within pkg/runtime/engine/singularity/config. There are also no unit tests on the parsing, and it's a little under-described.

In advance of sylabs/singularity#118 work, split the parsing functions into a separate file for clarity, and add basic testing. Add a couple of comments, and constants to make things a bit clearer. No real logic changes here - code was just moved.