Util.pm: add new sendmail_pipe and mailer_pipe utility functions
Currently /usr/bin/mail is hard-coded in many places as the
executable to use when sending mail messages.
As a first step to eliminating this, introduce a new sendmail_pipe
function that opens a pipe to a new sendmail process with the passed
in arguments. Although this change hard-codes /usr/sbin/sendmail
as the executable, this is a first step to making it configurable
as it only then need be changed in a single location.
Next introduce a mailer_pipe function that uses sendmail_pipe to
simulate basic support for /usr/bin/mail semantics (only the -s
option and recipients are supported).