man,spec: introduce ifcfg-dhcp man page

Covers DHCLIENT and DHCLIENT6 variables.
man: reference ifcfg-dhcp5 and remove template reference from ifcfg5
man: remove template reference from ifup8
man: remove template reference from ifcfg-wireless5
Merge pull request #435 from kmroz/ifcfg-dhcp

man: introduce ifcfg-dhcp man page covering DHCLIENT and DHCLIENT6 vars