agent.sh: Added $returnValue to check client errors during the agent update

This allows the verification of no Internet connection error during the
update process (as soon as the client isn't able to check the agent's
version).

Changed the test(1) outside checkUpdate() from "-eq 1" to "-ne 0".

Changed test(1) inside checkUpdate() to return 0 if it's the first
execution of the agent.

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Signed-off-by: Josiney de Souza <josineys@c3sl.ufpr.br>