After r281643 an #ifdef IFT_FOO preprocessor directive returns false,

After r281643 an #ifdef IFT_FOO preprocessor directive returns false,
since types became a enum C type.  Some software uses such ifdefs to
determine whether an operating systems supports certain interface type.
Of course, such check is bogus. E.g. FreeBSD defines about 250 interface
types, but supports only around 20.
However, we need not upset such software so provide a set of defines. The
current set was taken to suffice the dhcpd.

Reported & tested by:	Guy Yur <guyyur gmail.com>