[Patch, libgfortran] Inquire internal unit within child dtio procedure

[Dominique d'Humières]

Dear Jerry,

> Committed as trivial.

Many things are trivial unless noted otherwise! The patch causes the following failures:

FAIL: gfortran.dg/inquire_13.f90   -O0  execution test
FAIL: gfortran.dg/inquire_15.f90   -O0  execution test
FAIL: gfortran.dg/inquire_6.f90   -O0  execution test
FAIL: gfortran.dg/inquire_9.f90   -O0  execution test
FAIL: gfortran.dg/large_unit_2.f90   -O0  execution test
FAIL: gfortran.dg/make_unit.f90   -O0  execution test
FAIL: gfortran.dg/negative_unit.f   -O0  execution test
FAIL: gfortran.dg/negative_unit_int8.f   -O0  execution test
FAIL: gfortran.dg/pr20950.f   -O0  execution test

see, e.g., https://gcc.gnu.org/ml/gcc-testresults/2016-10/msg00375.html.

Also the test gfortran.dg/dtio_15.f90 succeeds even with revision r240768 reverted.

Thanks for working on the issue while waiting for the four character fix!

Dominique