Improve digimode configuration

[zcsahok]

Some minor improvements in digital mode configuration.

1. start in digimode if FLDIGI or GMFSK is set. Digimode has no explicit keyword like SSBMODE and this mode was set only by RTTYMODE. As the digital mode is not necessarily RTTY, requiring RTTYMODE for digimode is not logical.

2. DIGI_RIG_MODE can use any rig mode supported by Hamlib This information is transparent to TLF, so let Hamlib parse it.

3. new DIGITAL_MODE configuration to specify the mode used in ADIF export, also improving Cabrillo output reusing and extending modem_mode from RTTYMODE. For Cabrillo this controls whether RY (for RTTY) or DG (other digital mode) is output.

4. updated man page on these topics