seccomp-util: include @sandbox in @default

Every services and containers should be able to protect their users and
limit the impact of security bugs thanks to the security syscalls
provided by seccomp and Landlock.  The goal of these syscalls is to
improve security with additional restrictions.  They are designed to be
safely used by unprivileged (and then potentially malicious) users.

Remove the now-redundant "seccomp" entry for nspawn.

src/nspawn/nspawn-seccomp.c

@@ -84,7 +84,6 @@ static int add_syscall_filters(
                 { 0,                  "sched_rr_get_interval"        },
                 { 0,                  "sched_rr_get_interval_time64" },
                 { 0,                  "sched_yield"                  },
-                { 0,                  "seccomp"                      },
                 { 0,                  "sendfile"                     },
                 { 0,                  "sendfile64"                   },
                 { 0,                  "setdomainname"                },

src/shared/seccomp-util.c

@@ -318,6 +318,7 @@ const SyscallFilterSet syscall_filter_sets[_SYSCALL_FILTER_SET_MAX] = {
                 .name = "@default",
                 .help = "System calls that are always permitted",
                 .value =
+                "@sandbox\0"
                 "arch_prctl\0"      /* Used during platform-specific initialization by ld-linux.so. */
                 "brk\0"
                 "cacheflush\0"