exec: add exec plugin
exec: add tag "exec_rc" for line with command return code, remove empty line displayed before
exec: display command return code after output (stdout/stderr)
exec: remove error prefix in case of unexpected end of a command
exec: implement options -signal/-kill/-killall in command /exec
exec: add list of signal names in /help exec
exec: replace number by id in /help exec
exec: add option "-set" in command /exec
exec: fix typos in /help exec
exec: update plugin API version
exec: add option "-bg" in command /exec
exec: add option "-del" in command /exec
exec: fix crash when giving bad id to command /exec -in
core: add missing \0 at the end of stderr buffer in hook_process
exec: add option exec.command.default_options
exec: add options "-n"/"-ns" in command /exec (display output in a new buffer)
exec: add missing completion of "-ns" in command /exec
exec: add option "-inclose" in command /exec
exec: add options "-buffer" and "-sw"/"-nosw" in command /exec
exec: automatically disable shell if command starts with "url:" (URL download)