libxl: child processes cleanups

libxl: child processes cleanups

Abolish libxl_fork.  Its only callers were in xl.  Its functionality
is now moved elsewhere, as follows:

* The "logging version of fork", which is what it was billed as, is now
  xl_fork, which also dies on failure.

* Closing the xenstore handle in the child is now done in
  libxl__ev_child_fork, which is the only remaining place where fork
  is called in libxl.

* We provide a new function libxl__ev_child_xenstore_reopen for
  in-libxl children to make the ctx useable for xenstore again.

* Consequently libxl__spawn_record_pid now no longer needs to mess
  about with its own xenstore handle.  As a bonus it can now just use
  libxl__xs_write.

Also, since we have now converted all the forkers in libxl, we can
always honour the fork_replacement childproc hook - so do so.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>