timesyncd: split privilege dropping code out of timesyncd so that we can make use of it from other daemons too

This is preparation to make networkd work as unpriviliged user.
units: remove CAP_SYS_PTRACE capability from hostnamed/networkd

The ptrace capability was only necessary to detect virtualizations
environments. Since we changed the logic to determine this to not
require priviliges, there's no need to carry the CAP_SYS_PTRACE
capability anymore.