Fix a misconfiguration of mail.

I  was  modifying  the  `mail-header-separator' variable.   I  did  it
because  mutt  buffer does  not  use  the  same separator  that  emacs
uses.  That setup  were screwing  with mu4e  and in  general iwth  the
default message-mode  setup. Since  I do  not use  mutt I  removed the
faulty configuration.

I also setup some mu4e features (header view now is more sensible).