sx: several minor reworks

Calling exit directly in the relevant trap instead of using an argument
to decide when to trigger an exit in the cleanup function.

Additionally the EXIT trap is ignored in the INT trap so that when
running under bash the EXIT trap does not call cleanup again.

As `xauth remove :n' does not trigger warnings or problems for nonextant
entries it is safe to set the cleanup traps prior to using `xauth add'.
This removes a very minor potential race condition between adding an
entry and the script being interrupted before cleanup traps have been
established.  Suggested by @tomty89

Finally, attempt to make the main comment a little less awkward.

sx

@@ -1,11 +1,11 @@
 #!/bin/sh --
-# sx - start an xserver
+# sx - start an xorg server
 # requires xauth Xorg /dev/urandom
 
 cleanup() {
-    if [ "$pid" ] && kill -0 "$pid" 2> /dev/null; then
-        kill "$pid"
-        wait "$pid"
+    if [ "$server" ] && kill -0 "$server" 2> /dev/null; then
+        kill "$server"
+        wait "$server"
         xorg=$?
     fi
 
@@ -14,10 +14,6 @@ cleanup() {
     fi
 
     xauth remove :"$tty"
-
-    if [ "$1" = exit ]; then
-        exit "${xorg:-0}"
-    fi
 }
 
 stty=$(stty -g)
@@ -31,17 +27,16 @@ mkdir -p -- "$cfgdir" "$datadir"
 export XAUTHORITY="${XAUTHORITY:-$datadir/xauthority}"
 touch -- "$XAUTHORITY"
 
+trap 'cleanup; trap "" EXIT; trap - INT; kill -s INT "$$"' INT
+trap 'cleanup; trap "" EXIT; exit "${xorg:-0}"' EXIT HUP TERM QUIT
+
+# Xorg will return a USR1 signal to the parent process indicating it is ready
+# to accept connections if it inherited a USR1 signal with a SIG_IGN
+# disposition.  Consequently a client may be started directly from a USR1
+# signal handler and obviate the need to poll for server readiness.
+trap 'DISPLAY=:$tty "${@:-$cfgdir/sxrc}" & wait "$!"' USR1
+
 xauth add :"$tty" MIT-MAGIC-COOKIE-1 "$(od -An -N16 -tx /dev/urandom | tr -d ' ')"
-
-trap 'cleanup; trap - INT; kill -INT "$$"' INT
-trap 'cleanup exit' EXIT HUP TERM QUIT
-
-# Xorg will check whether it inherited a USR1 with a disposition of SIG_IGN and
-# use this state to reply back to the parent process with its own USR1.
-# This is done to indicate that the server is ready to accept connections.
-# Taking advantage of this feature allows launching the client directly from a
-# USR1 signal trap which obviates the need to poll for server readiness.
-trap 'DISPLAY=:$tty exec "${@:-$cfgdir/sxrc}" & wait "$!"' USR1
-
-(trap '' USR1 && exec Xorg :"$tty" -keeptty vt"$tty" -noreset -auth "$XAUTHORITY") & pid=$!
-wait "$pid"
+(trap '' USR1 && exec Xorg :"$tty" vt"$tty" -keeptty -noreset -auth "$XAUTHORITY") &
+server=$!
+wait "$server"