log: Introduce slilog_part; abolish log_if->logfn

log: Introduce slilog_part; abolish log_if->logfn

[v]Message provides a facility for sending messages to the system log
which are assembled out of pieces.  This is quite useful.  Generalise
it to other logger interfaces too:
  * Move the functionality out of vMessage into a new function
    vslilog_part.  Provide slilog_part too.
  * Move the assembly buffer: it was a static variable (used only
    for the system log); now it is a member of the log_if.  (Yes,
    the log_if, not the private state, because it applies for all
    loggers and is used by common code ie vslilog_part.)
  * Initialise log_if->buff[0] everywhere.
  * Rename LOG_MESSAGE_BUFLEN from MESSAGE_BUFLEN since now it
    has to live in secnet.h.

Also, remove log_if->logfn.  All the call sites use vlogfn.  Doing
this in this patch makes it easy to see that we haven't missed any
places where we should be initialising log_if->buff[0].

We currently have -Wunused ie -Wunused-functions, so for now we leave
the pointless definitions of the various loggers' logfns.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>