logger: use SCM_CREDENTIALS on LINUX only

logger: use SCM_CREDENTIALS on LINUX only

This is a build fix of FreeBSD and OSX. Basically we revert
the following commit for non-linux systems:
27a9eb53 "use --id as local socket credentials"

Note I could also compile it like this:

  #ifdef HAVE_SYS_UCRED_H
  # define _WANT_UCRED
  # include <sys/param.h>
  # include <sys/ucred.h>
  # define SCM_CREDENTIALS SCM_CREDS
  #endif

  [...]
  #ifdef _linux_
  cred->pid = ctl->pid;
  #endif
  [...]

... but don't know how to test whether it does what it
should.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>