pid1: pass unit name to seccomp parser when we have no file location

pid1: pass unit name to seccomp parser when we have no file location

Building on previous commit, let's pass the unit name when parsing
dbus message or builtin whitelist, which is better than nothing.

seccomp_parse_syscall_filter() is not needed anymore, so it is removed,
and seccomp_parse_syscall_filter_full() is renamed to take its place.