binary safety when dumping svs and ops

dump.c: print embedded control chars in names, esp. \0
add -Wno-unused-value also to clang++ or clang3
no strict 'syms'; check illegal names

Add a new strict check syms
Disallow illegal unparsable symbol and class names.

Esp. interesting are embedded \0 in classnames, which were until 5.16
silently ignored, and since 5.16 allowed. Since 5.16 names are
internally nul-safe, but roundtrips are not supported, and
such hidden payloads are useless for perl, are hard to detect and
may lead to security problems.