kill -SIGXXX works only in bash and not when the shell is e.g. dash, for
example:

	$ kill -SIGUSR1 1
	/bin/sh: 2: kill: Illegal option -S

This way log rotation for slapproxy.log & friends was not properly working.

Fix it by using `kill -XXX` form which is supported everywhere.

/reviewed-on nexedi/slapos!402