Use two tags instead of three for captures under iteration.

Use two tags instead of three for captures under iteration.

Before we simplified POSIX disambiguation by inserting missing
captures, using three tags to represent captures under iteration
was the only way: we needed to guard where the first iteration starts,
where the last iteration starts and where the last iteration ends.
Therefore we used three tags: opening, closing and orbit.

Now that we insert missing pieces of capture hierarchy, we no longer
need a special opening tag for captures under iteration: there always
is a higher-priority tag that guards the start of the first iteration.

Massive test updates are mostly due to the fact that orbit tag is no
longer fixed (fixed tags go in the end): ~100 out of 147 tests.
Some other ~45 tests have two tag variables flipped. There are 4 tests
with non-trivial changes. All updated tests pass POSIX test
(/test/posix_captures/.run/__run.sh).