Add ELAPSED process column

Add process columns showing the elapsed time since the process was
started.
Similar to STARTTIME, but shows the time passed since the process start
instead of the fixed start time of the process.

Closes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=782636

openbsd/OpenBSDProcess.c

@@ -108,6 +108,12 @@ const ProcessFieldData Process_fields[LAST_PROCESSFIELD] = {
       .description = "Time the process was started",
       .flags = 0,
    },
+   [ELAPSED] = {
+      .name = "ELAPSED",
+      .title = "ELAPSED  ",
+      .description = "Time since the process was started",
+      .flags = 0,
+   },
    [PROCESSOR] = {
       .name = "PROCESSOR",
       .title = "CPU ",