Add missing setuser.8

Upstream-bug: https://github.com/bruceg/daemontools-encore/issues/60

--- /dev/null
+++ b/setuser.8
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+.TH setuser 8
+.SH NAME
+setuser \- runs another program under a specified account's environment.
+.SH SYNOPSIS
+.B setuser
+.I account
+.I child
+.SH DESCRIPTION
+.I account
+is a single argument.
+.I child
+consists of one or more arguments. 
+
+.B setuser
+sets
+.BR $GID ,
+.BR $UID ,
+.BR $SHELL ,
+.BR $HOME ,
+and
+.BR $USER
+to
+.IR account 's
+gid number, uid number, login shell, home directory, and user name, respectively.
+It changes the working directory to
+.IR account 's
+home, and sets its uid and gid to
+.IR account 's
+uid and gid, with supplementary groups set up.
+It then runs
+.IR child .
+
+.B setuser
+cannot be run by anyone other than root.
+.SH EXIT CODES
+.B setuser
+exits 111 if it cannot find a UNIX account named
+.IB account ,
+if it cannot setup the environment, change directory, setgid, setuid, or run
+.IR child .
+Otherwise its exit code is the same as that of
+.IR child .
+.SH SEE ALSO
+envdir(8),
+envini(8),
+envuidgid(8),
+fghack(8),  
+multilog(8),
+pgrphack(8),
+readproctitle(8),
+setlock(8),
+setuidgid(8),
+softlimit(8),
+supervise(8),
+svc(8),
+svok(8),
+svscan(8),
+svscanboot(8),
+svstat(8),
+tai64n(8),
+tai64nlocal(8)