Retire all remaining IRIX and SINIX support. [skip appveyor]

Retire all remaining IRIX and SINIX support. [skip appveyor]

AC_LBL_LIBRARY_NET in tcpdump is out of date, study the current
requirements and copy from libpcap only what is really necessary.  This
works as expected, for example:

Linux, NetBSD and Solaris 11.4:

checking for gethostbyaddr... yes

illumos:

checking for gethostbyaddr... no
checking for gethostbyaddr in -lsocket... yes

Haiku:

checking for gethostbyaddr... no
checking for gethostbyaddr in -lsocket... no
checking for gethostbyaddr in -lnetwork... yes