Update dead links in reference manual.

doc/reference_manual.texi:
	Update dead WWW links (mainly to stuff that used
	to be hosted at CSSE.)

	Mark up URLs using @uref, so that we get actual
	links in the HTML version of the manual.
Fix URL markup in the user's guide.

doc/user_guide.texi:
	As above.
Fix the documentation of --feedback-file.

compiler/options.m:
doc/user_guide.texi:
	`--feedback-file' requires an argument.

	Fix a spot where an option name is not quoted /
	marked up.