r12531: 'make quicktest' was taking 15 minutes on my system due to failing DNS

r12531: 'make quicktest' was taking 15 minutes on my system due to failing DNS
lookups in load_interfaces(). The reason was my eth0 interface was
down, and it was being interpreted as a DNS name.

This patch changes load_interfaces() to happening automatically when
interfaces are first needed instead of on the startup of every samba
binary. This means that (for example) ldbadd doesn't call
load_interfaces(), which means no slow DNS lookups.

I also reduced the number of static globals in interface.c to 1, and
changed from malloc to talloc

When you want to force a reload of the interfaces list, you now call
unload_interfaces(), which means the next call that needs the
interfaces list will reload it