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/test/meson.build

@@ -400,6 +400,10 @@ tests += [
          [],
          []],
 
+        [['src/test/test-procfs-util.c'],
+         [],
+         []],
+
         [['src/test/test-unaligned.c'],
          [],
          []],

src/test/test-procfs-util.c

@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include <errno.h>
+
+#include "log.h"
+#include "procfs-util.h"
+
+int main(int argc, char *argv[]) {
+        uint64_t v;
+        int r;
+
+        log_parse_environment();
+        log_open();
+
+        assert_se(procfs_tasks_get_current(&v) >= 0);
+        log_info("Current number of tasks: %" PRIu64, v);
+
+        assert_se(procfs_tasks_get_limit(&v) >= 0);
+        log_info("Limit of tasks: %" PRIu64, v);
+        assert_se(v > 0);
+        assert_se(procfs_tasks_set_limit(v) >= 0);
+
+        if (v > 100) {
+                uint64_t w;
+                r = procfs_tasks_set_limit(v-1);
+                assert_se(IN_SET(r, 0, -EPERM, -EACCES, -EROFS));
+
+                assert_se(procfs_tasks_get_limit(&w) >= 0);
+                assert_se((r == 0 && w == v - 1) || (r < 0 && w == v));
+
+                assert_se(procfs_tasks_set_limit(v) >= 0);
+
+                assert_se(procfs_tasks_get_limit(&w) >= 0);
+                assert_se(v == w);
+        }
+
+        return 0;
+}