Userland: Add an "adjtime" utility

It's a thin userland wrapper around adjtime(2). It can be used
to view current pending time adjustments, and root can use it to
smoothly adjust the system time.

As far as I can tell, other systems don't have a userland utility
for this, but it seems useful. Useful enough that I'm adding it to
the lagom build so I can use it on my linux box too :)

Base/usr/share/man/man1/adjtime.md

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+## Name
+
+adjtime - print remaining system clock adjustment, and optionally set it
+
+## Synopsis
+
+```sh
+adjtime [options...]
+```
+
+## Description
+
+`adjtime -s delta_seconds` will smoothly adjust the system time by slowing it
+down (if `delta_seconds` is negative) or speeding it up (if `delta_seconds` is
+positive) by a fraction of a second. The larger `delta_seconds` is, the longer
+this takes. If `delta_seconds` is set and a previous time adjustment is in
+progress, the remaining adjustment is canceled. That is, if `adjtime -s 1` is
+called, and then `adjtime -s 1` is called again later when only 0.3s of the
+first `adjtime` call have been applied yet, the clock is adjusted by 1.3
+seconds total, not by 2 seconds.
+
+`adjtime` also prints the remaining system clock adjustment.
+
+## Options
+
+* `-s delta_seconds`, `--set delta_seconds`: Adjust system time by
+  `delta_seconds`. Must be superuser.
+
+## Examples
+
+```sh
+# adjtime -s 4.2
+4.2
+# sleep 1 && adjtime
+4.1
+```