fortran: [PR126116] Internal file I/O corrupts NEWUNIT state

Before this fix, Performing an internal file I/O
(such as WRITE to a character variable) incorrectly corrupts
the internal unit management table in libgfortran.

This patch adds a bool to the gfc_unit structure to allow
tracking whether a gfc_unit was created for purposes of
internal I/O to avoid the breakage.

	PR libfortran/126116

libgfortran/ChangeLog:

	* io/io.h (struct gfc_unit): Add bool for tracking.
	Fix some white space problems.
	* io/unit.c (close_unit_1): Add a declaration for this
	function so it can be called before it has been defined.
	Use the new bool.
	(is_internal_reserved): New helper function. Returns true
	only if the unit is not null and the internal unit kind is zero
	meaning its not a character string.
	(find_unit): Use the new helper function
	(find_or_create_unit): Use the new helper.
	(get_unit): Set the new bool.
	(close_unit): Update the close_unit_1 argument list.
	(close_units): Likewise.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr126116.f90: New test.

(cherry picked from commit 11be877aea73b0b26638078908a42dd0f97d81d7)