Implemented RTS/CTS handshaking (-C/--rtscts vs. -x/--xonxoff)

Implemented RTS/CTS handshaking (-C/--rtscts vs. -x/--xonxoff)
Implemented disable of +CMGF PDU/text mode autodetection (-M/--smsmode)
Implemented additional preceding SMSC modes for compat. (-P/--pdusmscmode)
 - "count-in" is default, "count-out" precedes w/o counting, "none" discards
 - "count-out" needed at least by recent Siemens M20 firmwares
 - autodetected when not forced by user
 - hardware provided by courtesy of Radek Kadner, ATS Praha
Fixed: total retrycount - now really (maxretry), not (maxretry+1) as before
Main communication routine devcmd() fortified and made foolproof
"AT+CSCA?" is now allowed to omit ADDR_* type
 - needed at least by recent Siemens M20 firmwares
Fixed timestamps by telling 'not known DST' (instead of 'no DST' as before)
PDU mode SMS receive works now (fatal bugs, never tested before)