configure: Stop avoiding -Wformat-zero-length.

configure: Stop avoiding -Wformat-zero-length.

Debian likes to enable -Wformat-zero-length, even over our code trying to
disable it.  It isn't too hard to make our code warning-free against this
option, so this commit both stops disabling it and fixes the warnings.

The first fix is to change set_subprogram_name() to take a plain string
instead of a format string, and to adjust its few callers.  This fixes one
warning since one of those callers passed in an empty string.

The second fix is to remove a test for ovs_scan() against an empty string.
I couldn't find a way to avoid a warning for this test, and it isn't too
valuable in any case.

This allows us to drop filtering for -Wformat from the Debian rules file,
so this commit removes it.

Signed-off-by: Ben Pfaff <blp@nicira.com>