rpmTagFoo() getters take arbitrary integers, not rpmTag enums

rpmTagFoo() getters take arbitrary integers, not rpmTag enums
- Another step to clean up the enum braindamage...
- rpmTag values are the only values that will be found, but it doesn't
  mean the values passed are rpmTag enum members - eg anything from
  a header is not. rpmTagGetValue() does come close to returning true
  enums, but there's RPMDBI_PACKAGES and then RPMTAG_NOT_FOUND, neither
  of which are actually tags.
- Introducing a new rpmTagVal typedef. It's just an alias for the
  "lowlevel" rpm_tag_t type but visually more in line with the other
  rpmTagFoo bits - to be used with the "higher level" tags whereas
  rpm_tag_t is the "physical" type. Not that it makes any difference...