seccomp: allow turning off of seccomp filtering via env var

Fixes: #17504

(While we are it, also move $SYSTEMD_SECCOMP_LOG= env var description
into the right document section)

Also suggested in: https://github.com/systemd/systemd/issues/17245#issuecomment-704773603

src/nspawn/nspawn-seccomp.c

@@ -186,7 +186,7 @@ int setup_seccomp(uint64_t cap_list_retain, char **syscall_allow_list, char **sy
         int r;
 
         if (!is_seccomp_available()) {
-                log_debug("SECCOMP features not detected in the kernel, disabling SECCOMP filterering");
+                log_debug("SECCOMP features not detected in the kernel or disabled at runtime, disabling SECCOMP filtering");
                 return 0;
         }