Add a vcs module, with Git and Subversion support.

[tee3]

Proposed changes

See boostorg/build#135

Types of changes

See boostorg/build#135

Put an x in the boxes that apply

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Documentation content changes
• [ ] Other (please describe):

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] I searched the discussions
• [x] I searched the closed and open issues
• [x] I read the contribution guidelines
• [ ] I added myself to the copyright attributions for significant changes
• [x] I checked that tests pass locally with my changes
• [ ] I added tests that prove my fix is effective or that my feature works
• [x] I added necessary documentation (if appropriate)

Further comments

This stalled as it requires changes to the engine as described by @swatanabe in the original pull request. I am not sure if this is feasible or not.

[tee3]

@grafikrobot

1. Do you think this would be a useful addition? I saw that there is package manager support in B2 now, but I have never figured out how to use it, nor Conan. Does this package manager support make this module obsolete?
2. If this is still something that might be useful, have the changes to LOCATE described in the original pull request been done? Can they be?
3. Do you have a better design for this that I could try to implement?
4. I am by no means a B2 expert, but I have managed to build some useful modules for my purposes (interfacing to the Xilinx tools, toolsets for TI compilers, etc.). I'm sure they could all be better, but I have learned a lot in the process. I would be willing to do the work on this module to make it work within the B2 targets, etc. if there was a chance it would be useful.
5. There is a very small subset of this vcs module that could be useful. It can generate a version string looking at the underlying vcs repository. I use this to generate versions in my professional work and it is really useful and seems to work properly in all cases. I would be willing to break that out to start if you feel it is useful.
6. If you have a better idea on how to accomplish this or feel it is not useful, I am fine with closing the pull request