Improve C# and Java shell script launchers.

The generated shell script to launch C# executables did not use absolute
paths.  The C# and Java shell scripts were missing some variable
quoting, leading to problems with paths containing spaces.

compiler/compile_target_code.m:
	Assign the script directory to DIR and use that to launch the
	CLI interpreter.

	Quote the CLI_INTERPRETER and executable filename.

compiler/module_cmds.m:
	Quote Mercury-generated CLASSPATH in case the directory contains
	spaces.

	Make DIR absolute in case the script is invoked from a
	subdirectory.