[flang][runtime] Fix fixed-width field internal wide character input (#74683)

There was some confusion about units (bytes vs characters) in the
handling of the amount of input remaining in fixed-width formatted input
fields. Clarify that any variable or parameter counting "remaining"
space in a field in the I/O runtime is always in units of bytes, and
make it so where it wasn't.

Fixes the bug(s) in
llvm-test-suite/Fortran/gfortran/regression/char4_iunit_2.f03, although
the test still won't pass due to its dependence on gfortran's
list-directed output spacing.