Backports the bugfix for the off-by-one error in rewrites for
compatibility with Varnish 5.2.

Currently tested with re2 through 2018-04-01.

The terminating null byte for the C string result is written at
the index given by std::string.size() for the C++ string. It had
been written at one past that index.

WS_Release() must then ensure that the terminating null byte is
within the allocated portion of workspace.

Includes a workaround for the problem in Varnish issue #2508.

And fix duplicate targets in src/Makefile.

Ensure that the retrieved object was saved via set.add().

Checked with gcc/g++ and clang/clang++.

Otherwise builds can fail due to partially written files from vmodtool
running on another target.

Take advantage of the fact that the array of indexes returned from
the vre2 call is now sorted (but still using a linear search).

for some early versions of re2.

Also get rid of an unnecessary #include "config.h".

Reduce code duplication for set.string() and .backend().

vcl.h must come before vrt.h for the VMOD event function typedef.

See Varnish issue #2449