Build: makefiles: simplify silent rules usage

make output is terser with --enable-silent-rules at configure time or make V=0
at make run-time. This simplifies our handling of such silent rules, mainly by
using AM_V_at and AM_V_GEN appropriately.

This gets rid of AM_V_IMG (which we never defined) and AM_V_ASCII and AM_V_XSL
(which weren't particularly useful), and adds AM_V_SCHEMA (to replace the
half-hearted attempt at RNG handling). Our PCMK_quiet now just silences stdout,
not stderr.