Remove obj.last_use
Rework a bit of the build system for vmod docs

Move the vmod man pages to man/ and build and ship the vmod_director
page.  Also make sure we use the vmod_std docs from the .vcc file, not
the old copy in docs/
Reset busyobj state to BOS_INVALID in VBO_GetBusyObj

Fixes: #1458
Document the version marker
split DEVELOPER_CFLAGS to multiple lines for legibility
dont try to build documentation for tools we cant build
DEVELOPER_CFLAGS:
- use -fstack-protector only if both compiler and linker support it
- use -Wno-missing-field-initializers vs -Wno-extra depending on compiler support
- -Wno-pointer-sign only if compiler supports it
- add -Wno-address to avoid "the address of ‘buf’ will never be NULL" warning in
  cache_backend_cfg.c (and in other places where REPLACE() is used with local
  buf variable)
Move obj.hits to objhead, make it read-only
Remove obsolete comment about obj.hits
Fix the three obj.{proto,reason,status} variables to match reality.
Update.. somewhat
Add missing dependency
Typo
Update after moving vmod docs to man/