log: enhance syslog logging using CEE format

This patch enables to use syslog as log target in addition to the
default.  The logs are sent in CEE format (http://cee.mitre.org/).
This logging can be disabled using compile time option by

./configure --disable-syslog
(or)
rpmbuild glusterfs.tar.gz --without syslog

The framework provides two api

void gf_openlog (const char *ident, int option, int facility);
void gf_syslog (int error_code, int facility_priority, char *format, ...);

consumers need to call gf_openlog() prior to gf_syslog() like the way
traditional syslog function calls.  error_code is mandatory when using
gf_syslog().  For example,

gf_openlog (NULL, -1, -1);
gf_syslog (GF_ERR_DEV, LOG_ERR, "error reading configuration file");

Using syslog, admin is free to configure logger to
* reduce repeated log messages
* forward logs to remote logger
* execute a command on certain log pattern
* alert people for certain log pattern by email, snmp etc
* and many more

Change-Id: Ibacbcbbc547192893fc4a46b387496b622e4811f
BUG: 928648
Signed-off-by: Bala.FA <barumuga@redhat.com>
Reviewed-on: http://review.gluster.org/4915
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>