Improvements to linux dosemu support

The hardcoded external.bat file has been moved to a templated /exec/external.bat which can be modified by the sysop. ENV vars are in there to be able to customize as needed. emusetup.bat is no longer needed, but still supported. The actual dosemu command line is now stored in /exec/dosemulaunch.ini and can be customized for fossil or i/o command lines. the keystroke is now only sent for i/o, and only sent as \n instead of \r which would trigger undesired behavior in certain programs. The above 2 files can also be placed in door dirs to override. ansi.com has been removed as it was not needed. default external.bat includes cmd lines for share, x00, etc. (user will still need to provide these in xtrn/dosutils).