Rewrite and fix specifiers handling

Rewrite and fix specifiers handling

Make sure that we accept the right set; kill ad-hackery around checks
for banned combinations.  Instead of that we keep a bitmap describing
what we'd already seen (with several extra bits for 'long long' and
for keeping track of can't-combine-with-anything stuff), check and
update it using the values in ..._op and keep track of size modifiers
more or less explicitly.

Testcases added.  A _lot_ of that used to be done wrong.

Note that __attribute__((mode(...))) got more broken by this one;
the next several changesets will take care of that.

One more thing: we are -><- close to getting rid of MOD_SPECIFIER bits
for good.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christopher Li <sparse@chrisli.org>