exec: SystemCallLog= directive

With new directive SystemCallLog= it's possible to list system calls to be
logged. This can be used for auditing or temporarily when constructing system
call filters.

---
v5: drop intermediary, update HASHMAP_FOREACH_KEY() use
v4: skip useless debug messages, actually parse directive
v3: don't declare unused variables with old libseccomp
v2: fix build without seccomp or old libseccomp

src/core/execute.h

@@ -302,6 +302,9 @@ struct ExecContext {
         int syscall_errno;
         bool syscall_allow_list:1;
 
+        Hashmap *syscall_log;
+        bool syscall_log_allow_list:1; /* Log listed system calls */
+
         bool address_families_allow_list:1;
         Set *address_families;