nss-systemd,user-util: add a way how synthesizing "nobody" can be turned off

nss-systemd,user-util: add a way how synthesizing "nobody" can be turned off

This is quite ugly, but provides us with an avenue for moving
distributions to define the "nobody" user properly without breaking legacy
systems that us the name for other stuff.

The idea is basically, that the distribution adopts the new definition
of "nobody" (and thus recompiles systemd with it) and then touches
/etc/systemd/dont-synthesize-nobody on legacy systems to turn off
possibly conflicting synthesizing of the nobody name by systemd.