procfs-util: add APIs to get consumed CPU time and used memory from /proc

This is preparation for emulating the "usage_usec" keyed attribute of
the "cpu.stat" property of the root cgroup from data in /proc. Similar,
for emulating the "memory.current" attribute.

src/basic/procfs-util.h

@@ -3,6 +3,12 @@
 
 #include <inttypes.h>
 
+#include "time-util.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);
+
+int procfs_cpu_get_usage(nsec_t *ret);
+
+int procfs_memory_get_current(uint64_t *ret);