build: add --enable-lto to enable Link Time Optimization (bgo #741140)

Takes about 3x as long to build with gcc 4.8, but gcc 4.9
is supposed to speed that up considerably.

Name           Before   After   Saved
-------------------------------------
NetworkManager  1734744  1689728   3%
libnm           1263536   808816  36%
nm-iface-helper  931136   906496   3%
libnm-util       441264   437168   1%
libnm-glib       297064   292960   2%

https://bugzilla.gnome.org/show_bug.cgi?id=741140
core: clean up half-done IP operations when re-entering NEED_AUTH state (bgo #741342)

When the device decides it needs re-auth during IP config and returns
to the NEED_AUTH state, make sure we clean up any half-done IP operations
since they will be re-started after auth is completed and the
IP_CONFIG state is re-entered.

https://bugzilla.gnome.org/show_bug.cgi?id=741342