lib: Fix privs when using HAVE_CAPABILITIES

lib: Fix privs when using HAVE_CAPABILITIES

If your daemon does not need any special privileges
and you are compiling with HAVE_CAPABILIES, the
zprivs->change pointer will end up NULL due
to the way zprivs_caps_init.  So as a check
let's add a NULL check for zprivs->change
and set it to a function that will do nothing.

This change prevents a crash if you raise privileges
when your daemon needs no special privileges.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>