[flang] Fix INQUIRE(PAD=) and (POSITION=) for predefined units

The predefined units were not being initialized with FORM='FORMATTED',
so INQUIRE(PAD=) was failing if no I/O had already been done.

INQUIRE(POSITION=) was returning 'REWIND' on stdin/stdout (which
is somewhat defensible from the definition, and is what Intel Fortran
does), but most implementations return 'ASIS'.  Change the runtime
to return 'REWIND' only for positionable external files, but 'ASIS'
for terminals, sockets, &c.

Differential Revision: https://reviews.llvm.org/D114028