Rework zed_notify_email for configurable PROG/OPTS

Rework zed_notify_email for configurable PROG/OPTS

This commit reworks the zed_notify_email() function to allow
configuration of the mail executable and command-line arguments.

ZED_EMAIL_PROG specifies the name or path of the executable responsible
for sending notifications via email.  This variable defaults to "mail".

ZED_EMAIL_OPTS specifies command-line options passed to ZED_EMAIL_PROG.
The following keyword substitutions are performed:
- @ADDRESS@ is replaced with the recipient email address(es)
- @SUBJECT@ is replaced with the notification subject
This variable defaults to "-s '@SUBJECT@' @ADDRESS@".

ZED_EMAIL_ADDR replaces ZED_EMAIL (although the latter is retained
for backward compatibility).  This variable can contain multiple
addresses as long as they are delimited by whitespace.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3634
Closes #3631