Remove the home-grown my_ulonglong type. It was meant to always be 64-bit
and was confusingly distinct from the ulonglong type, so just replace it
with uint64_t.

Note that since it is used in mysql.h for some return functions, a fair
amount of client software declares variables of type my_ulonglong.
Thus, we keep a legacy typedef, so that this software will not break
on compilation. (There is no ABI change, as we just replace one 64-bit type
with another.)

There is one more snag; since uint64_t is frequently defined to be of type
unsigned long, which is taken to be distinct from unsigned long long
(the old definition), certain software that takes these types and fed them
to printf directly will need adjustment to format strings, or get a new
compiler warnings. A quick sample over open-source software (see e.g.
https://codesearch.debian.net/search?q=printf.*mysql.*rows) indicates that
this pattern isn't all that common to begin with, and that most code that
does it uses %d, which already gave a warning in the old code. Note that
the warning is harmless and easily fixed by using PRIu64 (from C99) instead
of llu, or a cast back to long long int (like a lot of software from the given
sample already does).

Change-Id: If49eaf6cfaf3dda874f2a465068b481216b243c6