On startup log subcomponent versions

[djs55]

To make understanding bug reports easier, the log output will now contain the versions of

• uwt
• hvsock
• hostnet

[djs55]

@yomimono @samoht @avsm any objections to this wonderful PR? :)

[samoht]

LGTM

@dbuenzli is it possible to watermak using more complex strategies (instead of calling opam config subst + sed manually?)

[dbuenzli]

Currently the best you can use is String (see docs).

Now the problem with this is that to define the value with the result of a program invocation, you'd need to call this on each pkg.ml invocation.

I know I had at a certain point Value of unit -> (string, R.msg) result) in the design but I'm not sure why I eventually left it out. If you feel this can be useful file an issue.