syslog plugin: Initialize syslog as soon as possible.

Don't wait for the init callback to be called -- we might receive log messages
earlier than that.
syslog plugin: Don't log DEBUG messages for now.
utils os: Added sdb_mkdir_all() helper function.

This function recursively creates a directory (similar to 'mkdir -p' on the
command line).
frontend: Ensure that the base directory of a UNIX socket exists.
utils os: Don't enforce the mode on existing directories in mkdir_all().

This behaves badly when trying to create base directories like /var/run/ ;-)
Don't free dirname()'s return value.

It may point to static memory.
sock_test: Use mkstemp() instead of tmpnam().
connection_test: Use mkstemp() instead of tmpnam().
utils os: Added sdb_remove_all() to recursively remove a path.
os_test: Added tests for os_mkdir_all() and os_remove_all().