[flang][runtime] Don't pad CHARACTER input at end of record unless PAD='YES'

When formatted CHARACTER input runs into the end of an input record,
the runtime usually fills the remainder of the variable with spaces,
but this should be conditional, and not done when PAD='NO'.

And while here, add some better comments to two members of connection.h
to make their non-obvious relationship more clear.

Differential Revision: https://reviews.llvm.org/D125053