Attributes lack leading and trailing underscores

[PHHargrove]

This problem is distinct from issue #94, but manifests on the same input.

Given the following input:

#ifdef __GNUC__
__attribute__((__format__ (__printf__, 1, 2)))
#endif
static void fatal(const char *s, ...) {
    [...body...]
}

upc2c is producing the following output:

static void fatal(const char *s, ...) __attribute__((format(printf, 1, 2))) {
    [...body...]
}

The issue here is the use of format and printf without the leading and trailing __. This usage is "correct", but the form with the underscores is preferred because it lies in the namespace reserved to the implementation and thus should not be subject to problems if, for instance, printf() or format are names of macros.