Revert "get rid of ipv4_in_subnet"

This reverts commit e2fa48f1955481ed207a1b92808f9b47350e1d2d.

Conflicts:
	src/lib/network.inc

reason: removing ipv4_in_subnet breaks at least one feature :
filtering phonebook searches from phones. No phone can do phonebook
searches, they are all rejected with code 403 Forbidden.
remove gateway validation for interface form
fix ipv4_in_subnet for 64 bits
remove bit shift right

Is a no-operation.
remove unused functions bitmask_to_netmask_ipvX