emulator: fix notify_ring

notify_ring should not use information from waiting call
voicecall: Don't set indicators during transitions
sim900: Add ussd and voice call support
voicecall: Improve transitions check

Indicators should not be updated if:
- multiple separate calls are active at same time
- a conf call and a call are active at same time
- multiple separate calls are held at same time
- a conf call and a call are held at same time
- a conf call has call in active and held state
call-forwarding: Remove cf_list_clear()

Use g_slist_free_full() instead.
call-forwarding: Inline get_query_next_cf_cond()
gisi: Remove includes of <glib/gtypes.h>
isimodem: Remove includes of <glib/gtypes.h>
emulator: Force indicator event implementation
voicecall: Force callheld update after calls swap

In HFP spec, a callheld indicator update should be sent after swapping
calls, even if it stays to 1 (AG has both active and held calls).
build: Require glib 2.28
swmodem: Add Sierra Wireless modem driver
swmodem: Add Sierra Wireless DirectIP support
sierra: Add support for DirectIP connections
udev: Only detect Sierra Wireless devices with DirectIP
swmodem: Add support for IPv6 and IPv4v6 contexts
sim: Use quoted string with AT+CRSM data parameter

Fix issue with some modems preventing to update elementary files
(speedup, ZTE, huawei, MBM)
sim: Fix up various coding style violations
sim: Don't use sprintf unnecessarily

Using sprintf for a single quote is quite wasteful
huaweimodem: Fix AT^DDSETEX=2 timing for voice calls

Voice output serial port is enabled on some Huawei models (e.g. E169)
without problems, but for example on E173u-2 it is never enabled
during an incoming call. There might also be other Huawei models
having the same issue.

I traced the issue down to "^DDSETEX" AT command, which is used
to notify the device to start streaming audio. It seems that Ofono
sends this command too early on incoming calls. The command should
always be sent *after* the dial "D" or answer "A" command. The patch
fixes this behavior and afterwards voice will also work on E173u-2.