revised U command: fail if no argument is provided

 * there is no reasonable default value for U
 * omitting the parameter for U might be a frequent programming error
 * U can be colon-modified now, in which case it may be used
 * to check for the presence of arguments in macros
allow "-EC" command even though we do not generally imply the sign prefix
fixed EC command for negative line ranges
fixed EG command: make sure to reset the register argument on termination,
else the next EC command will not work as expected or even crash
support new "~" conditional: useful for implying default parameters in macros