Fix op/taint.t failure on Windows with 64-bit gcc-4.8.0 from MinGW-w64

MinGW-w64's 64-bit gcc now uses the SEH C++ exception model rather than
SJLJ or Dwarf2 (see http://mingw-w64.sourceforge.net/download.php).  For
such gccs, op/taint.t will need libgcc_s_seh-1.dll rather than
libgcc_s_sjlj-1.dll or libgcc_s_dw2-1.dll.  Fixes perl #121699.

Likewise, libwinpthread-1.dll is necessary if gcc was compiled with POSIX
threading and linked with the libwinpthread library, which is the case for
the gcc-4.8.2 toolchain that is likely to be used with the Strawberry Perl
5.20.x series.  Thanks to kmx for this.
Upgrade to Encode-2.60

This upgrade includes the blead customizations from 080a704606 and
9e9002efd1 (amongst other changes) so that Perl 5.20.0 can ship with an
official CPAN release of Encode.
[perl #121771] Revert the new warning for ++ on non- /\A[a-zA-Z]+[0-9]*\z/

This failed as in it was producing:

  Argument "123abc" treated as 0 in increment (++) at -e line 1.

when the user incremented that value (which is a lie).

This reverts commits 8140a7a801e37d147db0e5a8d89551d9d77666e0 and
2cd5095e471e1d84dc9e0b79900ebfd66aabc909.

I expect to revert this commit, and add fixes, after 5.20 is released.

Conflicts:
	pod/perldiag.pod
Revert "Upgrade to Encode-2.60"

This reverts commit dc396cc29397b262d3cc1473ade4229c84e82ca3.
PATCH: [perl #121778]:BBC KAMIPO/autobox-String-Inflector

When I wrote commit 645de4ceb9a7cbcff21ef7df3a5f5eb36259571e,
and its predecessor commit, I wrongly thought that this code used a
count of bytes rather than characters.  I no longer remember why.
My previous experience was that a lot of code in the regex system talked
about characters in the documentation, when in fact bytes were what was
really meant.  The comments had never been properly updated to account
for UTF-8

But this code in fact is supposed to use a character count, and this
commit changes it to do so.
Bring a few lines in perlxstut.pod under 80 cols
Fix remaining too-long verbatim lines in perlxstut.pod
re-apply: Upgrade to Encode-2.60

Commit dc396cc293 was reverted by 547245703 due to issues under g++.
This commit re-applies the original commit; the next commit will fix those
issues.

-----

This upgrade includes the blead customizations from 080a704606 and
9e9002efd1 (amongst other changes) so that Perl 5.20.0 can ship with an
official CPAN release of Encode.
Fix Encode 2.60 with g++

The recently added Encode 2.60 won't compile with g++, due to assigning
a const char * const pointer to a char* struct field.

The intent of the code itself is a bit unclear, but it appears to be
to set SvPVX as a read-only alias of a const string, using the SvLEN()=0
trick to avoid it being freed.

Fix the g++ builds by casting away the constness, and add some asserts and
comments to make it less unclear what's going on.
Revert "Upgrade Devel::PPPort from 3.21 to 3.22"

This reverts commit 8e5dcc37de4ab79d8ec6f30798947ae97355ff2a, since
the SvREFCNT_dec_NN is bad (leaks).
perlre: Update obsolete example

/foo{4,3}/ now emits a message, contrary to what the pod claims.  Use a
different example that doesn't emit a message
[perl #121505] include -fwrapv by default for GCC 4.3 and later
Implement "max waste" thresholds to avoid problems with COW and deliberately overallocated pvs

COW does not play nicely with "preallocate" algorithms.

More specifically code like sv_gets() wants to preallocate a large
buffer into $_ for performance reasons.

Prior to COW this was just fine. When someone assigned $_ to a
less volatile variable only the used portion of the buffer was copied,
and the extended buffer would be reused by sv_gets() and all was well.

With COW however this process is foiled. The preallocated buffer
get shared, and then when $_ is updated the buffer is dropped from $_,
leaving the other SV holding ownership of the overallocated buffer,
and causing sv_gets() to allocate a new buffer entirely. This process
would then repeat consuming time and lots of memory.

This patch introduces a "wastage" check to COW. When decided if we
should COW a string we look at the ratio and difference of SvCUR(sv)
and SvLEN(sv), which represent the "actual string length" and the
"allocated string length". When the difference exceeds a hard threshold,
or when the ration exceeds a designated factor then we do not COW.

This means that strings with large overallocations are not COWed.
Exactly how this works out in practice, where SvGROW() *always*
overallocates, is an open question.

See: https://rt.perl.org/Ticket/Display.html?id=121796

This patch also slightly tweaks SvGROW() not to do roundup on the first
allocation of the pv. Odds are good that the initial request realy does
want exactly what they expected. (SvGROW contrary to what the name
suggests is used for bother *extended* the size of a pv, and
initializing it the first time.)
document sv_gets() internals

sv_gets() contains some very dense code. while trying to rewrite I
learned a lot about it worked, and Nicholas asked me to update
the comments. So here they are. Hope this saves someone some heartache
trying to understand this code.
sv.c: fix closing comment

replace /* with */

it didn't matter because it was immediately followed by another comment
incorporate perl5190delta into perl5200delta
incorporate perl5191delta into perl5200delta
incorporate perl5192delta into perl5200delta
incorporate perl5193delta into perl5200delta
incorporate perl5194delta into perl5200delta