Fortran: Implement f_c_string function.

Fortran 2023 has added the new intrinsic function F_C_STRING to
convert fortran strings of default character kind to a null
terminated C string.

Contributions from Steve Kargl, Harald Anlauf, FX Coudert, Mikael Morin,
and Jerry DeLisle.

	PR fortran/117643

gcc/fortran/ChangeLog:

	* check.cc (gfc_check_f_c_string): Check arguments of f_c_string().
	* gfortran.h (enum gfc_isym_id): New symbol GFC_ISYM_F_C_STRING.
	* intrinsic.cc (add_functions): Add the ISO C Binding routine f_c_string().
	Wrap nearby long line to less than 80 characters.
	* intrinsic.h (gfc_check_f_c_string): Prototype for gfc_check_f_c_string().
	* iso-c-binding.def (NAMED_FUNCTION): Declare for ISO C Binding
	routine f_c_string().
	* primary.cc (gfc_match_rvalue): Fix comment that has been untrue since 2011.
	Add ISOCBINDING_F_C_STRING to conditional.
	* trans-intrinsic.cc (conv_trim): Specialized version of trim() for
	f_c_string().
	(gfc_conv_intrinsic_function): Use GFC_ISYM_F_C_STRING to trigger in-lining.

gcc/testsuite/ChangeLog:

	* gfortran.dg/f_c_string1.f90: New test.
	* gfortran.dg/f_c_string2.f90: New test.