Use IN_foo() macros from sys/netinet/in.h inplace of handcrafted code

Use IN_foo() macros from sys/netinet/in.h inplace of handcrafted code

There are a few places that use hand crafted versions of the macros
from sys/netinet/in.h making it difficult to actually alter the
values in use by these macros.  Correct that by replacing handcrafted
code with proper macro usage.

Reviewed by:		karels, kristof
Approved by:		bde (mentor)
MFC after:		3 weeks
Sponsored by:		John Gilmore
Differential Revision:	https://reviews.freebsd.org/D19317