email: Split sender into both sendmail's -F and -f

Use -F for the name and -f for the address, instead of passing the
composite "name <address>" to -f.  Only the address is used in the
SMTP envelope [1,2], so most mailers will probably ignore -F.  For
example, Postfix only uses it when there is no 'From' header in the
message itself [3].

The old behavior broke some sendmail implementation that assumed the
whole -f argument was an address.  I haven't noticed one of these
sendmail implementions myself, but they'll create envelope senders
like:

  MAIL FROM:<"foobar <abc>" <foo@bar>>

when we only want:

  MAIL FROM:<foo@bar>

[1]: http://tools.ietf.org/html/rfc2821#section-3.3
[2]: http://tools.ietf.org/html/rfc2821#section-4.1.1.2
[3]: http://www.postfix.org/sendmail.1.html