[swss.sh/syncd.sh] Trap only on EXIT (sonic-net#11590)

When using trap on SIGTERM the script will not react to the SIGTERM signal sent while a child is executing.
I.e, the following script does not react on SIGTERM sent to it if it is
waiting for sleep to finish:

```

trap "echo Handled SIGTERM" 0 2 3 15

echo "Before sleep"
sleep inf
echo "After sleep"
```

Instead, trap only on EXIT which covers also a scenario with exit on
SIGINT, SIGTERM.

Signed-off-by: Stepan Blyschak <stepanb@nvidia.com>

files/scripts/swss.sh

@@ -38,7 +38,7 @@ function lock_service_state_change()
 
     exec {LOCKFD}>${LOCKFILE}
     /usr/bin/flock -x ${LOCKFD}
-    trap "/usr/bin/flock -u ${LOCKFD}" 0 2 3 15
+    trap "/usr/bin/flock -u ${LOCKFD}" EXIT
 
     debug "Locked ${LOCKFILE} (${LOCKFD}) from ${SERVICE}$DEV service"
 }

files/scripts/syncd_common.sh

@@ -3,11 +3,11 @@
 #
 # common functions used by "syncd" scipts (syncd.sh, gbsyncd.sh, etc..)
 # scripts using this must provide implementations of the following functions:
-# 
+#
 # startplatform
 # waitplatform
 # stopplatform1 and stopplatform2
-# 
+#
 # For examples of these, see gbsyncd.sh and syncd.sh.
 #
 
@@ -25,7 +25,7 @@ function lock_service_state_change()
 
     exec {LOCKFD}>${LOCKFILE}
     /usr/bin/flock -x ${LOCKFD}
-    trap "/usr/bin/flock -u ${LOCKFD}" 0 2 3 15
+    trap "/usr/bin/flock -u ${LOCKFD}" EXIT
 
     debug "Locked ${LOCKFILE} (${LOCKFD}) from ${SERVICE}$DEV service"
 }