Use `sendmail` command instead of `mailx` to send email

Avoids functional shortcomings of heirloom-mailx (which is the new
default mailx in Ubuntu).  In addition to having different cmdline
args, heirloom-mailx has no way to specify the Content-type, which
is a prerequisite for sending HTML email.

Change-Id: I73988670775c46ddad3651af6ff04333bbe4624c
3 files changed