Split ParseBindpath from config get/setters and add --mount, from sylabs 319 & 324

[DrDaveD]

This pulls in sylabs prs

• Split ParseBindPath from config Get/Setters & add basic tests sylabs/singularity#319
  and
• Add --mount flag for long form mount specifications sylabs/singularity#324
  which resolved issue
• Long form --mount to support special chars in bind mount paths etc. sylabs/singularity#118

Did not include the sylab's PR's addition of a StringArray flag, and instead used the cmdline.StringArray type added in #6008.

The original pr descriptions were:

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 #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.

Add a --mount flag, and corresponding environment variable that can be used to specify bind mounts in type=bind,source=<src>,destination=<dst>[,options...] format. This improves CLI compatibility with other runtimes, and allows binding paths containing : and , characters (using CSV style escaping like docker).

A general goal is to achieve as much compatibility as possible with https://docs.docker.com/storage/bind-mounts/#choose-the--v-or---mount-flag

* Only `type=bind` is supported, and inferred if not specified.

* Multiple binds can be specified with multiple `--mount` flags, or `\n` delimited strings in the `SINGULARITY_MOUNT` env var.

* `source/src`, `destination/dst/target`, `ro/readonly` are synonyms

* The singularity specific `image-src=<path>` and `id=<int>` options for bind mounting out of an image file are supported by `--mount`, as they are for `--bind/-B`.

* `--mount` support has been added for action commands and build.

Implementation is via parsing of --mount spec strings into a BindPath struct. This parsing is tested in unit tests. Since the functionality of different BindPath configurations is already verified in e2e tests with --bind flags, e2e testing of --mount is restricted to limited additional cases covering single and double --mount

Note also that CLI flag handling was extended to allow StringArray flags, where values are not split into a slice on ','. This was a requirement for the --mount spec syntax.