seccomp: add new seccomp_init_conservative() helper

This adds a new seccomp_init_conservative() helper call that is mostly just a
wrapper around seccomp_init(), but turns off NNP and adds in all secondary
archs, for best compatibility with everything else.

Pretty much all of our code used the very same constructs for these three
steps, hence unifying this in one small function makes things a lot shorter.

This also changes incorrect usage of the "scmp_filter_ctx" type at various
places. libseccomp defines it as typedef to "void*", i.e. it is a pointer type
(pretty poor choice already!) that casts implicitly to and from all other
pointer types (even poorer choice: you defined a confusing type now, and don't
even gain any bit of type safety through it...). A lot of the code assumed the
type would refer to a structure, and hence aded additional "*" here and there.
Remove that.

src/nspawn/nspawn-seccomp.c

@@ -135,15 +135,9 @@ int setup_seccomp(uint64_t cap_list_retain) {
                 return 0;
         }
 
-        seccomp = seccomp_init(SCMP_ACT_ALLOW);
-        if (!seccomp)
-                return log_oom();
-
-        r = seccomp_add_secondary_archs(seccomp);
-        if (r < 0) {
-                log_error_errno(r, "Failed to add secondary archs to seccomp filter: %m");
-                goto finish;
-        }
+        r = seccomp_init_conservative(&seccomp, SCMP_ACT_ALLOW);
+        if (r < 0)
+                return log_error_errno(r, "Failed to allocate seccomp object: %m");
 
         r = seccomp_add_default_syscall_filter(seccomp, cap_list_retain);
         if (r < 0)
@@ -171,12 +165,6 @@ int setup_seccomp(uint64_t cap_list_retain) {
                 goto finish;
         }
 
-        r = seccomp_attr_set(seccomp, SCMP_FLTATR_CTL_NNP, 0);
-        if (r < 0) {
-                log_error_errno(r, "Failed to unset NO_NEW_PRIVS: %m");
-                goto finish;
-        }
-
         r = seccomp_load(seccomp);
         if (r < 0) {
                 log_error_errno(r, "Failed to install seccomp audit filter: %m");