lang.opt: Add -fmax-errors= option.

lang.opt: Add -fmax-errors= option.

* lang.opt: Add -fmax-errors= option.
* gfortran.h (gfc_option_t): Add max_errors element.
* options.c (gfc_init_options): Set max_errors default value
to 25.
(gfc_handle_options): Assign -fmax_errors value to
gfc_option.max_errors.
* error.c (gfc_increment_error_count): New function, which
also checks whether the error count exceeds max_errors.
(gfc_warning): Use it.
(gfc_warning_now): Use it.
(gfc_notify_std): Use it.
(gfc_error): Use it.
(gfc_error_now): Use it.
(gfc_error_check): Use it.

From-SVN: r118615