so tell make in lib/libvcc how to invoke it.

Exposed by 062234c1: Now code changes
(as when switching branches) may require a rebuild in lib/libvcc only

... for a simple make clean, in particular not vmod_abi.h.

Otherwise, building from a trunk dist tarball will give a NOGIT
VMOD_ABI_Version after a "make clean", while the correct version
is in place for a simple make after unpacking the tarball.

In addition, instead of always initializing a transaction's storage
backend using the round-robin selection, pick Transient when we know
in advance that beresp will be uncacheable.

Closes #2533

We only need to transmit the ones where we differ from rfc7540's initial
values.

Of a VSM segment, we map the least number of pages from the offset into the file
covering the segment length, aligned to page boundaries. For calculating
relative pointers into the mapping and the length, we need to deduce the initial
page boundary offset.

Fixes #2541

Some specialcasing is warranted to get sensible messages.

Fixes #2532

stdin-fd is dead, it takes more than "break;" to get out of a
switch-in-a-loop.