build-sys: Add new libsystemd-shared private library

build-sys: Add new libsystemd-shared private library

Link as many binaries as possible with it, to save storage space.

Preserve the static libshared and libbasic for use in libraries, nss
modules and udev.

Libraries need to be static in order to avoid polluting the symbol
namespace.

Udev needs to be static so downstream can avoid strict version dependencies
with the systemd package, and this can complicate upgrade scenarios.