util: Allow set_subprogram_name() to take a printf() format string.

util: Allow set_subprogram_name() to take a printf() format string.

This will be convenient in an upcoming commit.

I had to add -Wno-format-zero-length to suppress a GCC warning about a
zero-length format string in this monitor_daemon() call:
    set_subprogram_name("");
I don't know what that warning is good for anyway, and I guess the Clang
developers don't either because Clang didn't warn.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>