Merge branch 'es/send-email-sendmail-alias'

Merge branch 'es/send-email-sendmail-alias'

"git send-email" learned to handle more forms of sendmail style
aliases file.

* es/send-email-sendmail-alias:
  send-email: further warn about unsupported sendmail aliases features
  t9001: add sendmail aliases line continuation tests
  t9001: refactor sendmail aliases test infrastructure
  send-email: implement sendmail aliases line continuation support
  send-email: simplify sendmail aliases comment and blank line recognizer
  send-email: refactor sendmail aliases parser
  send-email: fix style: cuddle 'elsif' and 'else' with closing brace
  send-email: drop noise comments which merely repeat what code says
  send-email: visually distinguish sendmail aliases parser warnings
  send-email: further document missing sendmail aliases functionality