ptx: fix some integer overflow bugs

ptx: fix some integer overflow bugs

Problem reported by Lukas Zachar at:
http://bugzilla.redhat.com/1482445
* src/ptx.c (line_width, gap_size, maximum_word_length)
(reference_max_width, half_line_width, before_max_width)
(keyafter_max_width, truncation_string_length, compare_words)
(compare_occurs, search_table, find_occurs_in_text, print_spaces)
(fix_output_parameters, define_all_fields):
Use ptrdiff_t, not int, for object offsets and sizes.
(WORD, OCCURS): Use ptrdiff_t, not short int.
(WORD_TABLE, number_of_occurs, generate_all_output):
Prefer ptrdiff_t to size_t where either will do.
(total_line_count, file_line_count, OCCURS, fix_output_parameters)
(define_all_fields):
Use intmax_t, not int, for line counts.
(DELTA): Remove.  All uses changed.
(OCCURS, find_occurs_in_text, fix_output_parameters):
Use int, not size_t, for file indexes.
(tail_truncation, before_truncation, keyafter_truncation)
(head_truncation, search_table, define_all_fields)
(generate_all_output):
Use bool for booleans.
(digest_word_file, find_occurs_in_text):
Use x2nrealloc instead of checking for overflow by hand.
(find_occurs_in_text, fix_output_parameters, define_all_fields):
Omit unnecessary cast.
(fix_output_parameters): Don’t assume integers fit in 11 digits.
(fix_output_parameters, define_all_fields):
Use sprintf return value rather than calling strlen.
(define_all_fields): Do not rely on sprintf to generate a string
that may contain more than INT_MAX bytes.
(main): Use xstrtoimax, not xstrtoul.
Use xnmalloc to catch integer overflow.