Fixes parallel build failures when compilation of vmod_re2.c,
set.c or re2.c is attempted before vmodtool is finished

cf. commit 76403011

Adds the bugfix for the off-by-one error in rewrites.

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.

compile_fail.vtc evidently takes longer than 60s when run in a
docker container.

Also update the compatibility statement for RE2 -- compatible up
to and including 2018-04-01.

And use $Synopsis manual in the documentation -- we write our own
SYNOPSIS.

NULL is no longer possible as an ENUM value.

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.

vrt.h is included by cache.h, and may not be included after it.

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".