v.io/jiri: switch cl mail to multipart mode by default.
v.io/jiri: switch cl mail to multipart mode by default.

MultiPart: 2/2
Change-Id: Id859cac12e83fa6bb8d3b4d826d64d90526b5454

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools