PerlIO::scalar: silence some compiler warnings
PerlIO::via: fix compiler warning

In
    GV *gv = newGVgen(HvNAME_get(s->stash))

HvNAME_get() can return a null value (don't know whether it ever will in
these circumstances), while newGVgen() expects a non-null arg.

So calculate HvNAME_get() first, and bail if it's null.
SDBM_File: fix 'set but not used' warning

the NEXTKEY() method has a key arg, but sdbm_nextkey() doesn't.
So don't bother mapping the key SV arg to a datum_key type.
POSIX: silence some compiler warnings
threads: silence some compiler warnings.

From 5.8.9, the MGVTBL struct has a 'svt_local' member

Declare some vars volatile within a function that uses jongjmp

Mark the unused return value from PerlLIO_write() as void(), even
though it won't shut up gcc.
XS::Typemap: silence compiler warning.

xsubpp will give us a RETVAL var whether we use it not.