This also adds documentation for it in distributed-builds.md as that's
possibly the most common use-case for remote ssh stores
Support for `std::optional<uint16_t>` in Setting is also added since
setting a port is optional.
The line `#include "lix/libutil/strings.hh"` fixes that templates instanciations
in lix/libutil/config-impl.hh were using string utils without including
the header (why are they even there btw)
Change-Id: Id806c117c48cdf158d9d1cb1e639b0df31d9bf11
Sebastian Widua
82c7e76c9c