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

[klausler]

Summary

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.

flang/runtime/io-stmt.cpp

bool InquireUnitState::Inquire(
   case HashInquiryKeyword("POSITION"):
     if (unit().access == Access::Direct) {
       str = "UNDEFINED";
     } else {
       auto size{unit().knownSize()};
       auto pos{unit().position()};
       if (pos == size.value_or(pos + 1)) {
         str = "APPEND";
-      } else if (pos == 0) {
+      } else if (pos == 0 && unit().mayPosition()) {
         str = "REWIND";
       } else {
         str = "ASIS"; // processor-dependent & no common behavior
       }
     }
     break;
   case HashInquiryKeyword("READ"):
     str = unit().mayRead() ? "YES" : "NO";

flang/runtime/unit.cpp

UnitMap &ExternalFileUnit::GetUnitMap() {
   Terminator terminator{__FILE__, __LINE__};
   IoErrorHandler handler{terminator};
   UnitMap *newUnitMap{New<UnitMap>{terminator}().release()};
   bool wasExtant{false};
   ExternalFileUnit &out{newUnitMap->LookUpOrCreate(6, terminator, wasExtant)};
   RUNTIME_CHECK(terminator, !wasExtant);
   out.Predefine(1);
   out.SetDirection(Direction::Output, handler);
+  out.isUnformatted = false;
   defaultOutput = &out;
   ExternalFileUnit &in{newUnitMap->LookUpOrCreate(5, terminator, wasExtant)};
   RUNTIME_CHECK(terminator, !wasExtant);
   in.Predefine(0);
   in.SetDirection(Direction::Input, handler);
+  in.isUnformatted = false;
   defaultInput = &in;
   // TODO: Set UTF-8 mode from the environment
   unitMap = newUnitMap;
   return *unitMap;
 }
 
 void ExternalFileUnit::CloseAll(IoErrorHandler &handler) {
   CriticalSection critical{unitMapLock};