ltq-vdsl-app: add support for auto xfer_mode and auto line_mode

ltq-vdsl-app: add support for auto xfer_mode and auto line_mode

If xfer_mode is set to auto the vdsl_cpe_control daemon assumes that
ATM should be used for ADSL and PTM for VDSL.

xfer_mode and line_mode can be set to fixed value independantly from
each other.

The syntax for the tc_layer argument of vdsl_cpe_control is as follow:

-T<TcADSL>:<TcCfgUsADSL>:<TcCfgDsADSL>_<TcVDSL>:<TcCfgUsVDSL>:<TcCfgDsVDSL>

where TcADSL and TcVDSL can be: 1=ATM, 2=PTM/EFM, 4=Auto TC-Layer

and TcCfgUsADSL, TcCfgUsVDSL, TcCfgDsADSL, TcCfgDsVDSL can be:
1=64/65-octet encapsulation supported
2=64/65-octet encapsulation with pre-emption
3=64/65-octet encapsulation with short packets

Default: In case of no '-T' option is given, ADSL will be configured
in ATM and VDSL in PTM/EFM: -T1:0x1:0x1_2:0x1:0x1

The '-M' argument of dsl_cpe_control defines the initial DSL mode
(NextMode) for ADSL/VDSL multimode handling.

Possible Values: 0=API-default, 1=ADSL, 2=VDSL

Default: In case of no '-M' option is given, '0' (API-default) will
be selected.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>