parser should handle its own parallelism

As Seth Arnold explained on https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=782700#74 (slightly rephrasing): it would be good if calling the parser once per directory with profiles worked, e.g. apparmor_parser --replace /etc/apparmor.d/ (which already works, but without DTRT parallelism). This would avoid the need for distro initscripts & friends to implement xargs-based solutions, see e.g. https://sources.debian.net/src/apparmor/2.10-1/debian/lib/apparmor/functions/.

Another blocker to get rid of such ugliness seems to be LP: #1377338.

Both issues make it harder than needed to port current sysvinit initscripts to systemd.