rc: fix an infinite loop on tty close

if the tty is closed while the script is running,
it would previously go in an infinite loop trying
to read from and write to the now-defunct tty.

fixes: https://github.com/opnsense/core/issues/1240

src/etc/rc.initial

@@ -18,6 +18,13 @@ fi
 # endless loop
 while : ; do
 
+# We `set -e' to force exit if we encounter an error.
+# This is mainly useful in case we lose our tty (happens when
+# an ssh connection breaks, for example), in which case our stdout
+# is closed and the `echo' commands in the while loop will silently fail.
+# Failure to exit at that moment would lead to an infinite loop.
+set -e
+
 echo
 
 echo " 0) Logout                             7) Ping host"
@@ -32,6 +39,9 @@ echo
 read -p "Enter an option: " OPCODE
 echo
 
+# The scripts we'll call below may return non-zero, don't exit if they do
+set +e
+
 # see what the user has chosen
 case ${OPCODE} in
 0|exit|logout|quit)