hfpmodem: Fix crash with more than two calls

A periodic CLCC polling is started when there is an ongoing multiparty
call and a new call appears in the system. A simple way to reproduce
the crashing scenario is:

1. Place a call.
2. Place a second call.
3. Create a multiparty call with both calls.
4. Place a third call (incoming or outgoing does not matter).
5. Disconnect HFP from the modem.

Within the function ciev_callheld_notify, the AT+CLCC command is also
invoked, thus a new cyclic CLCC polling is started, and it overwrites
the timer resource identifier stored in voicecall_data.clcc_source.
This means that there are several timers doing the CLCC polling, but
only one of those is under control, i.e. it can be removed through its
source identifier, hence a timer source leak.

This has a fatal consequence when the HFP modem is disconnected. The
function hfp_voicecall_remove stops the timer that is under control
before freeing the voicecall_data struct. However there are other timers
that are still active and will execute its handler poll_clcc afterwards.
Inside poll_clcc the driver_data is accessed, which is already NULL.

A solution for this is to avoid starting a CLCC polling if there is
already one active, i.e. clcc_source is not 0. By doing this the
uncontrolled timers will not cycle forever.
atmodem: add vendor Quectel
udevng: add detection logic for Quectel modems
sim: query Quectel UC15 PIN retries with AT+QPINC?
atmodem: Minor style fixups
atmodem: add vendor u-blox
gprs: add support for u-blox +UREG URCs
plugins: add a new driver for Quectel UC15 modems
quectel: Remove unneeded VENDOR flags

Only the sim atom has QUECTEL specific code
quectel: Minor style cleanup
udevng: add detection logic for u-blox modems
sim: query u-blox PIN retries with AT+UPINCNT
atmodem: set the auth method for u-blox modems
plugins: new driver for u-blox SARA-U270 modems

This driver may also work (perhaps with more or less trivial changes)
with other u-blox modems (SARA, LISA, LEON) but this hasn't been tested.
ublox: Remove unneeded VENDOR flags

devinfo and netreg atoms don't use vendor specific behavior
ublox: Minor style cleanup
build: Avoid warnings about _FORTIFY_SOURCE

Gentoo (and others) define _FORTIFY_SOURCE=2 by default.  We redefine
it, resulting in an error / configure failure.
gdbus: Avoid reporting GDBusClient disconnect twice

No matter if disconnection was reported previously, g_dbus_client_unref()
was always calling service disconnect callback. This patch fix the
following scenario:
1) service disconnects from the bus
2) disconnect callback gets called
3) client calls g_dbus_client_unref(), disconnect callback is called
   again.
gdbus: Fix use after free

Refactor filter_data_remove_callback so that we do not iterate over
freed pointer.
Revert "gdbus: Don't include just added interfaces in GetManagedObjects"

This actually creates a problem since the code now does flush any pending
message on the queue any signal will be send right away before the
actual reply to GetManagedObjects which will not contain those interfaces.