Improve and fix memory management in C plaintext converter

* tta/C/main/list_macros.h (decl_alloc_fns, def_alloc_fns): add
init_to_() macro function definition that reallocates and initialize
to zero the newly allocated memory.

* tta/C/convert/convert_to_plaintext.c (clear_pending_text_list)
(release_count_context, add_top_pending_text, push_count_context)
(COUNT_CONTEXT_STACK, PENDING_TEXT_LIST, stream_final_result)
(plaintext_add_target_location, ensure_end_of_line)
(merge_pending_texts, replace_pending_text, move_pending_texts)
(align_lines, align_environment, text_heading)
(convert_to_plaintext_internal, plaintext_free_converter): keep
allocated count context and pending texts and reset TEXT to reuse the
allocated memory.  Add add_top_pending_text and use it to push a
pending text that can be an already allocated pending text reused.
Remove the second argument of push_count_context, the context pushed
can be reused, and can be accessed afterwards.  Change
merge_pending_texts such that the count context added is swapped with
the source count context, such that the associated allocated memory,
if any, is kept.  Add replace_pending_text and move_pending_texts to
add to parent pending texts without doing the swap as with
merge_pending_texts, but with destruction of the memory at the
destination, for the case of pending text memory not reused, for
multitable and aligned environments.  Rename destroy_count_context as
release_count_context.  Add clear_pending_text_list to reset the texts
in the rare case when it is not already done when getting the pending
texts, for empty text_heading and multitable prototypes.  Deallocate
the count contexts and pending texts in plaintext_free_converter.
Remove reset_count_context_stack, clear_pending_texts and
free_pending_texts.

* tta/C/convert/convert_to_plaintext.c
(plaintext_convert_line_new_context): leave the count context on the
stack and pass a pointer to the pending text.  Update callers.

* tta/C/convert/convert_to_plaintext.c (pending_to_text)
(collect_pending_texts_lines): reset number of pending text to 0, in
functions where the input pending texts are reset.

* tta/C/convert/convert_to_plaintext.c
(convert_to_plaintext_internal): add row_cell_lines to format_context
and separate cells_entry_locations, remove row_cell_counts.  Call
collect_pending_texts_lines when processing a cell, and add cell_idx
in format context to know the current cell index.

* tta/C/convert/convert_to_plaintext.c (release_count_context)
(align_lines, align_environment, convert_to_plaintext_internal) :
reset images and entry locations properly, free pending lines and
images lists.

* tta/C/convert/convert_to_plaintext.c (convert_to_plaintext_internal):
get top formatter again after conversion, in case it was reallocated.
Remove getting the top formatter when it cannot have changed.

* tta/C/convert/convert_to_info.c (info_header),
tta/perl/Texinfo/Convert/InfoNonXS.pm (_info_header): use new_formatter,
push_formatter and pop_formatter in info_header too, such that
the formatter switches even if it is not done first.  Move enum
formatter_type to header file.

* tta/C/convert/convert_to_info.c (info_output),
tta/perl/Texinfo/Convert/InfoNonXS.pm (output): call
plaintext_cache_node_names before any use of node names such that
there is no need to check if there are already node names cached.

* tta/C/convert/convert_to_plaintext.c
(convert_to_plaintext_internal): do not pass image depth for
displaymath.  Add ensure_end_of_line.

* tta/C/convert/convert_to_info.c (info_output),
tta/C/convert/convert_to_plaintext.c (plaintext_process_printindex)
(plaintext_conversion_finalization, plaintext_insert_image)
(convert_def_line): free extension variable, destroy element
after conversion, free text, always destroy parsed_def, pop last count
context.  Deallocate properly element_images.