Define static hostname ip4-localhost

This mirrors the corresponding static hostname that is defined for IPv6:

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback

These lines exist because there are some situations where you can only conveniently specify a hostname but you wish to force a particular protocol. For the same reason "ip4-localhost" is useful.

BTS bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=916446