chg: add --version-string

chg: add --version-string

I've included a patch that provides better compatibility with CSD on
ASA head ends. E.g. it allows to specify the version string that is
presented to the ASA. Previous to this patch, OC presents its own
version e.g. 0.7.8 but that could cause rejection on the head end if
it looks for a matching AC version string.

[dwmw2: All the library ABI support for the new function]

Signed-off-by: Ralph Schmieder <ralph.schmieder@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>