Redo location tracking. Instead of having just files track the

Redo location tracking.  Instead of having just files track the
line to return to when popping input, now all input blocks track
their current line.
* src/input.c (INPUT_STRING_WRAP, INPUT_FILE_INIT): No longer
needed.
(input_block): Have line and file storage for all input types, and
rename some members.
(input_change): New global flag.
(push_file, push_macro, push_string_init, push_wrapup): Store
location.
(push_string_finish, pop_input, pop_wrapup): Notice changes in
input blocks.
(peek_input): Adjust to new member names.
(next_char, next_char1): Adjust location if needed.
(skip_line): Simplify restoring location.
* doc/m4.texinfo (Location): Augment the test to catch line
location of expansion of multi-line arguments.
Reported by Stepan Kasal.