- $1 can never be empty or it's an rpm bug

- Get rid of $FIRST_ARG
$FIRST_ARG was probably introduced because the %service_* macros
were playing tricks on the shell positional parameters. This is bad
practice and error prone so let's assume that no macros should do
that anymore and hence it's safe to assume that positional
parameters remains unchanged after any rpm macro call.
All users of $FIRST_ARG should have been fixed by now and in most
cases the use of the variable was unneeded (since the macros don't
change the shell parameters) and thus confusing.
'net-snmp' has a different use of FIRST_ARG though as it tried to
fake an update during a package installation. Fortunately this could
have been fixed too.