C 1.42_66: fix XSLoader::load_file

use proper dlext string append. fixes most 5.16,5.18 XS tests
add more DynaLoader::dl_debug output (set PERL_DL_DEBUG=1)
Simplify xs stub definitions to one-liners with SvREFCNT_inc_simple_NN()
remove DynaLoader from static_in_core list, needs to be booted explicitly.
support user-defined static_ext modules (add themm to static_in_core)
XSLoader_load_file: untodo some passing tests

however not all yet
CC: fix destruction of static COW strings

also set SvLEN to 0.
use full name for @static_free
CC: fix -faelem POPi,POPs case

use *av_fetch (svp)