libgfortran: [PR105473] Fix checks for decimal='comma'.

	PR libfortran/105473

libgfortran/ChangeLog:

	* io/list_read.c (eat_separator): Reject comma as a
	seprator when it is being used as a decimal point.
	(parse_real): Reject a '.' when is should be a comma.
	(read_real): Likewise.
	* io/read.c (read_f): Add more checks for ',' and '.'
	conditions.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr105473.f90: New test.