Add official support for Xmodem-G (aka Qmodem-G)

xmodem_get_ack() now returns an int (ACK on success)
Use 3-second block 0/1 timeout to follow ymodem.doc example
Fix the use of xm.max_errors
Support (configurable) fallback from Ymodem-send to Xmodem-send
sexyz.ini can be used to disable 1024-recv, CRC-send and G-send support
Document more sexyz command-line options to enable legacy x/ymodem modes