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/basic/locale-util.c

@@ -367,7 +367,8 @@ const char *special_glyph(SpecialGlyph code) {
                         [BLACK_CIRCLE]       = "*",
                         [ARROW]              = "->",
                         [MDASH]              = "-",
-                        [ELLIPSIS]           = "..."
+                        [ELLIPSIS]           = "...",
+                        [MU]                 = "u",
                 },
 
                 /* UTF-8 */
@@ -381,6 +382,7 @@ const char *special_glyph(SpecialGlyph code) {
                         [ARROW]              = "\342\206\222",             /* → */
                         [MDASH]              = "\342\200\223",             /* – */
                         [ELLIPSIS]           = "\342\200\246",             /* … */
+                        [MU]                 = "\316\274",                 /* μ */
                 },
         };
 

src/basic/locale-util.h

@@ -48,6 +48,7 @@ typedef enum {
         ARROW,
         MDASH,
         ELLIPSIS,
+        MU,
         _SPECIAL_GLYPH_MAX
 } SpecialGlyph;