ModemManager: disable modem functionality for now

org.freedesktop.ModemManager changed to ModemManager1 as well as
its API.
To be implemented later.
Remove private tags target, rely on built-in target
Merge pull request #141 from olafhering/olh-ctags-in-current-branch
dhcp4: moved src/dhcp-lease.c to src/dhcp4/lease.c
dhcp6: moved status and option names to src/dhcp6
dhcp6: use a complete name for ia-address option
xml: added new xml_node_new_element_uint utility
dhcp6: moved dhcp6/duid.[ch] to src/duid.[ch]
addrconf: added nds-server,context,tree to the lease
addrconf: initial lease writting/parsing helpers
Merge pull request #140 from pwieczorkiewicz/modemsoff

ModemManager: disable modem functionality for now
addrconf: fixed a read/write tag inconsistence
addrconf: initial lease file read/write (disarmed)
Merge pull request #142 from mtomaschewski/dhcp-leases

addrconf: initial lease writting/parsing helpers
ModemManager: fixing a warning introduced in 7acddcda

Instead of disabling the modem manager init function call
(leads to unused function warning) set global opt_no_modem_manager
switch to TRUE.