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/test/test-procfs-util.c

@@ -3,15 +3,24 @@
 #include <errno.h>
 
 #include "log.h"
+#include "parse-util.h"
 #include "procfs-util.h"
 
 int main(int argc, char *argv[]) {
+        char buf[CONST_MAX(FORMAT_TIMESPAN_MAX, FORMAT_BYTES_MAX)];
+        nsec_t nsec;
         uint64_t v;
         int r;
 
         log_parse_environment();
         log_open();
 
+        assert_se(procfs_cpu_get_usage(&nsec) >= 0);
+        log_info("Current sytem CPU time: %s", format_timespan(buf, sizeof(buf), nsec/NSEC_PER_USEC, 1));
+
+        assert_se(procfs_memory_get_current(&v) >= 0);
+        log_info("Current memory usage: %s", format_bytes(buf, sizeof(buf), v));
+
         assert_se(procfs_tasks_get_current(&v) >= 0);
         log_info("Current number of tasks: %" PRIu64, v);