PerlIO_find_layer should not be using memEQ() off the end of the layer name.

PerlIO_find_layer was using memEQ() to compare the name of the desired layer
with each layer in the array of known layers. However, it was always using
the length of the desired layer for the comparison, whatever the length of
the name it was comparing it with, resulting in out-of-bounds reads.
In S_scan_heredoc(), the explicit test for '\n' duplicates the strNE().

PL_tokenbuf always starts with '\n', so a separate test of *s against '\n'
is duplicate work. Hence remove it, to make the code simpler and clearer.
In In S_scan_heredoc(), avoid memNE() reading beyond the end of s.

If the heredoc terminator we are searching for is longer than the bytes
remaining in s, then the memNE() would read beyond initialised memory.
Hence change the loop bounds to avoid this case, and change the failure case
below to reflect the revised end-of-loop condition.

It doesn't matter that the loop no longer increments shared->herelines,
because the failure case calls S_missingterm(), which croaks.
xs_init() must pass a static char* when creating &DynaLoader::boot_DynaLoader.

newXS() assumes that the passed pointer to the filename is in static storage,
or otherwise will outlive the PVCV that it is about to create, and hence that
it's safe to copy the pointer, not the value, to CvFILE.  Hence xs_init()
must not use an auto array to "store" the filename, as that will be on the
stack, and becomes invalid as soon as xs_init() returns.  The analogous bug
fix was made in universal.c by commit 157e3fc8c802010d in Feb 2006.

Spotted by compiling for ithreads with gcc 4.8.0's ASAN and running
dist/B-Deparse/t/deparse.t
perlapi: Document some macros
pad.c, S_cv_clone: Maintain the utf8-ness of the cloned cv

Because of a missing SvUTF8_on() in cv_clone(), these two were different:

    use utf8;
    eval "   sub foo ($;\x{30cd});"
    eval "my sub foo ($;\x{30cd});"

Because the lexical version would lose the UTF8 flag in the
prototype.
WIP: Make dump.c nul-and-UTF8 clean
Silence warnings in dump.c