v.io/jiri: add policy driven flags for merging environment variables.
v.io/jiri: add policy driven flags for merging environment variables.

- Also tidy up v23-profile env, list etc.

MultiPart: 8/11

Change-Id: Ib10a6a26b1128b3b912981c03db91ab38f97feb8

jxson/reader,vanadium/reader,jxson/reader,vanadium/reader,vanadium/reader,jxson/reader,jxson/reader