analyze: Add "timespan" command to dump time span in usec

This is useful for a couple of cases, I'm mostly interested in case #1:

1. Verifying "reasonable" values in a trivially scriptable way
2. Debugging unexpected time span parsing directly

Test Plan:

```
% build/systemd-analyze timespan 20
Original: 20
      μs: 20
   Human: 20us
% build/systemd-analyze timespan 20ms
Original: 20ms
      μs: 20000
   Human: 20ms
% build/systemd-analyze timespan 20z
Failed to parse time span '20z': Invalid argument
```

src/test/test-locale-util.c

@@ -65,7 +65,7 @@ static void test_keymaps(void) {
 
 #define dump_glyph(x) log_info(STRINGIFY(x) ": %s", special_glyph(x))
 static void dump_special_glyphs(void) {
-        assert_cc(ELLIPSIS + 1 == _SPECIAL_GLYPH_MAX);
+        assert_cc(MU + 1 == _SPECIAL_GLYPH_MAX);
 
         log_info("/* %s */", __func__);