fix segfault in parameter parser
FDWatchers are non copiable and non movable
Server: correctly handle Accepters lifetime

When exiting kakoune, ClientAccepters (now Server::Accepter) could
stay alive, which left an FDWatcher in the EventManager triggering
an assert. Now Server is handling their lifetime.
do not write the trailing nul char in send_commands

That triggered creation of a new client, followed by it's immediate
destruction due to the connection closing.
port git-tools.kak from bash to posix shell + posix awk
Remove bash dependency, we only need posix tools now for bundled scripts
Remove the shell option, always use /bin/sh
ShellManager: Change the env var detection regex to find more uses

The regex is now just '\bkak_\w+\b', no need for it to be looking
like a shell variable use.

This allows other tools that the shell to access Kakoune vars as
long as the name appears directly in the shell string.