oxenstored: Re-add ocaml syslog binding

This was lost in the OCaml xenstored log merge of 10/Oct/2011.

The binding isn't exported as a shared "log" library, instead it is kept
local to xenstored.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
oxenstored: enable logging via syslog, if specified in the config file.

To log to files the config file should contain:
   xenstored-log-file = /var/log/xenstored.log
   access-log-file = /var/log/xenstored-access.log

(These two files are still the built-in defaults. The log format is
unchanged.)

To log to syslog the config file should contain:
   xenstored-log-file = syslog:<facility>
   access-log-file = syslog:<facility>

where <facility> is the syslog facility to use (e.g. 'daemon' 'local2')

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
oxenstored: Allow oxenstored to use syslog at levels other than Debug

We now log different kinds of events at different levels. The convention
is now:

new/end_connection:    Debug
coalesce:              Debug
conflict:              Debug
commit:                Debug
regular ops:           Info
start/end_transaction: Debug
error (ENOENT):        Debug
error (any other):     Warn
watch:                 Info

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>