chg: don’t use printf with variable format strings

chg: don’t use printf with variable format strings

Except in very exceptional cases, the format string to printf should
never be variable (and for maximum clarity, it should always be
single-quoted even if it happens to contain no shell special
characters). This commit changes one format string from double to single
quotes and rewrites two more to use substitution sequences instead of
substituting variables directly into the format (which is dangerous if
they could contain ‘%’ characters).

With this change, the following search finds no results:

    git grep "printf (?:>&2 )? [^']"

Change-Id: Ieeae16c483a7e568cd5812260a6ac54375c33340