conf: Add --runas option, changing to given UID and GID if started as root

On some systems, user and group "nobody" might not be available. The
new --runas option allows to override the default "nobody" choice if
started as root.

Now that we allow this, drop the initgroups() call that was used to
add any additional groups for the given user, as that might now
grant unnecessarily broad permissions. For instance, several
distributions have a "kvm" group to allow regular user access to
/dev/kvm, and we don't need that in passt or pasta.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

passt.1

@@ -95,6 +95,13 @@ Log to standard error too.
 Default is to log to system logger only, if started from an interactive
 terminal, and to both system logger and standard error otherwise.
 
+.TP
+.BR \-\-runas " " \fIUID\fR|\fIUID:GID\fR|\fILOGIN\fR|\fILOGIN:GROUP\fR
+If started as root, change to given UID and corresponding group if UID is given,
+or to given UID and given GID if both are given. Alternatively, login name, or
+login name and group name can be passed.
+Default is to change to user \fInobody\fR if started as root.
+
 .TP
 .BR \-h ", " \-\-help
 Display a help message and exit.