maint/update_copyright_years.sh: implement concurrent execution

maint/update_copyright_years.sh: implement concurrent execution

Analogous to the way it is done in xlat/gen.sh.

* maint/update_copyright_years.sh (MAX_JOBS): New variable, initialize
it to double the CPU count.
<while [ -n "${1:-}" ]; do case "$1" in>: Add -j option parsing.
(jobs, pids): New variables.
<git ls-files -- "$@" | grep -vFx "$IGNORED_FILES" | while read f; do>:
Execute process_file in background, count background jobs and wait
if there are too many.