Go around calling the default process filter (reducing GC churn)

Go around calling the default process filter (reducing GC churn)

Instead of allocating strings and passing them to the filter, pass
the char buffer to a C function implementing the same logic.

* src/process.c (read_process_output_before_insert)
(read_process_output_after_insert):
New functions, extracted from internal-default-process-filter.
(Finternal_default_process_filter): Use them.
(read_and_insert_process_output): New function.  Use them.
(read_process_output_fast): New variable.
(read_process_output): Use it to choose how to insert (bug#66020).

* etc/NEWS: Mention the change.