ktest.pl: Add MAIL_PATH option to define where to find the mailer

The option MAIL_PATH lets the user decide how to find the mailer they are
using. For example, sendmail is usually located in /usr/sbin but is not
always in the path of non admin users. Have ktest look through the user's
PATH environment variable (adding /usr/sbin) as well, but if that's not good
enough, allow the user to define where to find the mailer.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

squash to mail exec

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>