slapo-auditlog: Add olcAuditlogNonBlocking to avoid blocking when logging to named...

slapo-auditlog: Add olcAuditlogNonBlocking to avoid blocking when logging to named pipes

The default behaviour of fopen() when called on a named pipe which does not have any reader, is to block, until a reader opens the pipe.  This blocks slapo-auditlog when it attempts to write output.  Depending on how critical the audit log  is, it may be preferable to discard audit log output and continue processing requests if there's no reader available.

For clarity the call to fopen() is removed and replaced with open()/fdopen(), allowing us to specify O_* flags as opposed to using fopen() or open()/fdopen(). 0666 are the base permissions used by fopen() when files are created.