Dear Fortranners,

the subject says it all: in some cases we emit redundant integer division
truncation warnings (2 or 4), where just one would have been sufficient.
This is solved by using gfc_warning instead of gfc_warning_now.

The testcase uses a suggestion by Andrew to verify that we get the
desired warning exactly once.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald