util-lib: add new procfs-util.[ch] API for dealing with tasks limits

As it turns out the limit on concurrent tasks on Linux nasty to
determine, hence let's appropriate helpers for this.

src/basic/procfs-util.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include <inttypes.h>
+
+int procfs_tasks_get_limit(uint64_t *ret);
+int procfs_tasks_set_limit(uint64_t limit);
+int procfs_tasks_get_current(uint64_t *ret);