* dynl.c, feature.c, filesys.c, fports.c, list.c, load.c,

* dynl.c, feature.c, filesys.c, fports.c, list.c, load.c,
net_db.c, sort.c, stacks.c, unif.c: Use SCM_WTA, SCM_MISC_ERROR
where possible.

* symbols.c (scm_sysintern0): Fixed the function name in a
scm_misc_error invocation.

* print.c (scm_simple_format): Do not need SCM_COERCE_SUBSTR, and
use scm_return_first to ward off latent GC bug that Mikael caught.

* async.c: Use SCM_VALIDATE_ASYNC_COPY one place where it wasn't
used before but should've been.