Assume that /proc/meminfo can be missing

Travis tests are failing, probably because /proc/meminfo is not available
in the test environment. The same might be true in some virtualized systems,
so just treat missing /proc/meminfo as a sign that hibernation is not
possible.

src/shared/sleep-config.c

@@ -172,7 +172,8 @@ static bool enough_memory_for_hibernation(void) {
 
         r = get_status_field("/proc/meminfo", "\nSwapFree:", &swapfree);
         if (r < 0) {
-                log_error("Failed to retrieve SwapFree from /proc/meminfo: %s", strerror(-r));
+                log_full(r == -ENOENT ? LOG_DEBUG : LOG_WARNING,
+                         "Failed to retrieve SwapFree from /proc/meminfo: %s", strerror(-r));
                 return false;
         }