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
```

1 files changed, 10 insertions, 0 deletions

diff --git a/man/systemd-analyze.xml b/man/systemd-analyze.xml
index 7aa10fc68e..15e95b0c8f 100644
--- a/man/systemd-analyze.xml
+++ b/man/systemd-analyze.xml
@@ -106,6 +106,12 @@
       <arg choice="plain">service-watchdogs</arg>
       <arg choice="opt"><replaceable>BOOL</replaceable></arg>
     </cmdsynopsis>
+    <cmdsynopsis>
+      <command>systemd-analyze</command>
+      <arg choice="opt" rep="repeat">OPTIONS</arg>
+      <arg choice="plain">timespan</arg>
+      <arg choice="plain" rep="repeat"><replaceable>SPAN</replaceable></arg>
+    </cmdsynopsis>
   </refsynopsisdiv>
 
   <refsect1>
@@ -253,6 +259,10 @@ NAutoVTs=8
     <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
     The hardware watchdog is not affected by this setting.</para>
 
+    <para><command>systemd-analyze timespan</command> parses a time span and outputs the equivalent value in microseconds, and as a reformatted timespan.
+    The time span should adhere to the same syntax documented in <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
+    Values without associated magnitudes are parsed as seconds.</para>
+
     <para>If no command is passed, <command>systemd-analyze
     time</command> is implied.</para>