pg_upgrade: Fix exec_prog API to be less flaky

pg_upgrade: Fix exec_prog API to be less flaky

The previous signature made it very easy to pass something other than
the printf-format specifier in the corresponding position, without any
warning from the compiler.

While at it, move some of the escaping, redirecting and quoting
responsibilities from the callers into exec_prog() itself.  This makes
the callsites cleaner.