This reverts commit 886c788c.

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.