Add a FIXME mentioning that there are many

Add a FIXME mentioning that there are many
unchecked integer overflows in this file.
(gnu_extensions, auto_reference, input_reference, right_reference,
ignore_case, initialize_regex, fix_output_parameters,
output_one_roff_line, output_one_text_line, output_one_dumb_line, main):
Use bool for booleans.
(SKIP_SOMETHING, compare_words, digest_break_file,
find_occurs_in_text, fix_output_parameters):
Use to_uchar instead of a caset.
(print_field): Rewrite to avoid cast.