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).