Refer to ZED's scripts as ZEDLETs

The executables invoked by the ZED in response to a given zevent
have been generically referred to as "scripts".  By convention,
these scripts have aimed to be /bin/sh compatible for reasons of
portability and comprehensibility.  However, the ZED only requires
they be executable and (ideally) capable of reading environment
variables.  As such, these scripts are now referred to as ZEDLETs
(ZFS Event Daemon Linkage for Executable Tasks).