check_udp: skip tests which rely on udp listen

solaris netcat does not listen in udp mode due to
http://sourceforge.net/p/netcat/bugs/27/
check_by_ssh: tests expand key and config path

some systems do not expand the path automatically.
check_ssh: added test case
check_dhcp: fix tests interface detection
check_dhcp: fix mac address and interface number detection on solaris

newer (or x86) solaris implementations have interface names like e1000g0
which includes numbers. So we reverse the interface number detection to
get the last number, instead of the first number.
check_disk: die with a reasonable error message if one mount point hits average value