Upgrade to C-Kermit 9.0.304 Dev.11

This commit upgrades the C-Kermit base from 9.0.304 Dev.09 to 9.0.304
Dev.11. The only change to the official Dev.11 codebase is in ckuus4.c
removing the display_demo reference (the Windows/OS2 codebase no longer
includes this variable)