connect: add os-version info exchange

This is to address the point discuted in:

gitlab-shell#609 (comment 1444982614)

From the cover letter:

For debugging and statistical purposes, it can be useful for Git servers to know the OS the client are using.

So let's add a new 'os-version' capability to the v2 protocol, in the same way as the existing 'agent' capability that lets clients and servers exchange the Git version they are running.

This sends the same info as git bugreport is already sending, which uses uname(2). It should be the same as what uname -srvm returns, except that it is sanitized in the same way as the Git version sent by the 'agent' capability is sanitized (by replacing character having an ascii code less than 32 or more than 127 with '.').