security: Use a reserved character for Blessings.String
security: Use a reserved character for Blessings.String

The character used prior to this commit (#) was not reserved, so
it could lead to confusion in debug output
(e.g., does the string "foo#bar" mean two blessings: foo and bar or
one blessing "foo#bar")

This commit changes to using a reserved character that can't appear
in blessing names anyway (",")

MultiPart: 1/2

Change-Id: I6dce4bfd3a69756fbed104f85cc775a22521fad9

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23