Build fixes for Solaris

Build fixes for Solaris

* Use __func__ and define it to __FUNCTION__ if not defined.
  it's the C99 standard.

* Tests for certain linking libraries (libnsl, libsocket, libresolv) (configure.ac)

* Defines INADDR_NONE  if not defined

* Search for <ucred.h> in addition to <sys/ucred.h> (configure.ac)

* Changes include of <sys/errno.h> to <errno.h>.
  I _think_ this should be safe on all platforms (and it's the standard)

* Changes local variable name s_addr to saddr in takeover.c.
  s_addr is a Single Unix Specification defined name.

* Checks if LOG_AUTHPRIV exists (it does not in Solaris)

Original patch by Magne Mæhre, applied with minor changes