shared: introduce cmsg_close_all() call

The call iterates through cmsg list and closes all fds passed via
SCM_RIGHTS.

This patch also ensures the call is used wherever appropriate, where we
might get spurious fds sent and we should better close them, then leave
them lying around.

src/shared/ask-password-api.c

@@ -475,6 +475,8 @@ int ask_password_agent(
                         goto finish;
                 }
 
+                cmsg_close_all(&msghdr);
+
                 if (n <= 0) {
                         log_error("Message too short");
                         continue;

src/shared/util.c

@@ -8110,3 +8110,13 @@ ssize_t string_table_lookup(const char * const *table, size_t len, const char *k
 
         return -1;
 }
+
+void cmsg_close_all(struct msghdr *mh) {
+        struct cmsghdr *cmsg;
+
+        assert(mh);
+
+        for (cmsg = CMSG_FIRSTHDR(mh); cmsg; cmsg = CMSG_NXTHDR(mh, cmsg))
+                if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS)
+                        close_many((int*) CMSG_DATA(cmsg), (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int));
+}

src/shared/util.h

@@ -1080,3 +1080,5 @@ void sigkill_wait(pid_t *pid);
 #define _cleanup_sigkill_wait_ _cleanup_(sigkill_wait)
 
 int syslog_parse_priority(const char **p, int *priority, bool with_facility);
+
+void cmsg_close_all(struct msghdr *mh);