build-sys: warn if builds are not byte-by-byte reproducible due to usage of __DATE__ and suchlike
core: refuse doing %h, %s, %U specifier resolving in PID 1

These specifiers require NSS lookups to work, and we really shouldn't do
them from PID 1 hence. With this change they are now only supported for
user systemd instance, or when the configured user for a unit is root.