Userland: Add watch tool

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

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+## Name
+
+watch - execute a program periodically
+
+## Synopsis
+
+`watch [-n interval] [-t] [-b] command`
+
+## Description
+
+Run a command full-screen periodically until interrupted, then return the
+aggregated error code.
+
+## Options
+
+* '-n': Interval between executions, in seconds. By default, the program is run every 2 seconds.
+* '-t': Don't print the title bar.
+* '-b': Beep each time the command exits with a non-zero status
+
+## Exit Values
+
+* 0 - Success
+* 1 - At least one invocation of the command failed or exited with a non-zero status
+
+## Examples
+
+```sh
+$ watch -n1 ls
+$ watch -t -- uname -a
+```