tree-wide: get rid of selinux_context_t (#3732)

https://github.com/SELinuxProject/selinux/commit/9eb9c9327563014ad6a807814e7975424642d5b9
deprecated selinux_context_t. Replace with a simple char* everywhere.

Alternative fix for #3719.

src/journal/journald-server.c

@@ -727,7 +727,7 @@ static void dispatch_message_real(
                                 *((char*) mempcpy(stpcpy(x, "_SELINUX_CONTEXT="), label, label_len)) = 0;
                                 IOVEC_SET_STRING(iovec[n++], x);
                         } else {
-                                security_context_t con;
+                                char *con;
 
                                 if (getpidcon(ucred->pid, &con) >= 0) {
                                         x = strjoina("_SELINUX_CONTEXT=", con);