cbuf: change cbuf_free to take cbuf_t instead of pointer

Update cbuf clients to free the allocated cbuf_t instead of buf pointer.
Now cbuf_free is symmetrical to cbufp_deref.
cstub: refactor cstub macros

Simplify the macro interface. Also save and restore input registers
around sysenter call.
Makefile.end: avoid cosnet in installed Makefile
vagrant support for composite virtualbox VM

Add support for using vagrant (www.vagrantup.com) to bootstrap
a working composite development environment. Include documentation
and a line in gitignore to ignore configuration files vagrant creates
when it executes.
Vagrant.md: direct users to install virtualbox
vagrant: use explicit ssh host and port
doc: update vagrant documentation
vagrant: go back to using DHCP
cbufp: refactor and format
tests: use MAX_SZ parameter instead of hard-coded value
cbufp: break dependency with cbuf

Break the dependency of using a cbuf to return the garbage-collection list
from cbufp_collect() by mapping a shared page between the cbufp manager
and each client component. The list is stored in a ring buffer that is
accessed via a ck_ring structure located at the start of the shared
page.