Avoid an undefined behavior of memcpy() in _tr_stored_block().

Allegedly the behavior of memcpy() is undefined if the source
pointer is NULL, even if the number of bytes to copy is zero.

Trying to tackle #2460

for i < 0, rlen could underflow. We are safe because of the check for
i < 0 further down, so this change is just a minor cleanup.

Fixes #2444

Together with the previous commit, this fixes potentially misleading
error messages.

Because we called vcc_path_dlopen on all vmod_path elements
irrespective of the kind of dlopen() error, the actual root cause
could be hidden.

Example:

dlerror: .../lib/libvmod_blob/.libs/libvmod_debug.so: cannot open shared object file

when the actual error was

dlerror: .../lib/libvmod_debug/.libs/libvmod_debug.so: undefined symbol:...

Addresses #2456 in a different way.

ensure wrk is always set and, for the backend side, move vfp_ctx setup to a
single place in vbf_stp_startfetch()

Fixes #2453

Fixes #2451

Previously, a FetchError in vbe_dir_getfd() was too general to be
useful. Now we log an error with the same identifier as the
corresponding statistic:

* unhealthy
* busy (max_connections reached)
* fail (no connection for real)

For these, we also log the backend name because we have not yet issued
a BackendOpen VSL.

A failed WS_Alloc is logged as "out of workspace".

We use an implicit rule to tell make how to turn vsc into c and
manually define the vsc files in Makefile.am. The implicit rule tells
automake to add .vsc to SUFFIXES, which in turn allows us to add the
actual vscs to varnishd_SOURCES.

To avoid triple configuration of the list of built VSC .c and .h
files, we generate these in configure. AC_SUBSTing files into _SOURCES
is forbidden by autoconf for a good reason: Adding sources to the
build should remain an explicit process to avoid accidentally
compiling in code just because someone had files with a magic name
lying around.

As the implicit .vsc -> .c rule does not allow for additional
dependencies, we add in the dependency to vsctool using the
auto-generated list of c files.

BUILT_SOURCES is used to ensure that, for default make targets,
headers generated from vscs are always present before the actual
varnishd sources are being build, of which some depend on vsc
headers. For subsequent builds, include dependency tracking takes
over.

The Makefile rules are deliberately simple to hopefully cover all make
flavours available on the planet. Notice that, for example, if we
limited ourselves to gmake, we could use much simpler substitution
rules to generate VSC_GEN_C and _H instead of the de-tour via
configure.

Portability is also the reason for renaming all vsc files.

recycle connections between protocols.

Should fix distcheck

someone(tm) might revisit this again later

Otherwise we might see garbled output files

StringIO does not exist any more in python3, yet requiring 2.7 would
not pave the path forward, so try to be compatible with both.

Works for me on Python 2.7.9 and Python 3.4

I would appreciate if someone more fluent in serpentinous programming
language reviewed and/or rewrote this.

These are versions of H2_Send_Get/H2_Send_Rel that can be called while
already holding the lock.

Maintain stream 0 window limits via h2->req0.