Fix buglet in interactive help system

Closes #516

Turns out it isn't necessary to "eat" the newline character that ended
the user's typing of

help "somestring";

any more.

Removed trailing whitespace from all .sml and .sig files.

This affects over 900 files and was done using emacs's delete-trailing-whitespace
function in batch mode. Building the system with Poly/ML and Moscow ML seems to
work, so I'm hoping these changes don't break anything. Please complain if
they do!

Fix an error that was preventing the Poly help system from finding
signature files to browse. (This caused by the fact that
TextIO.openIn raises IO.Io, not OS.SysErr. Contrast with Moscow ML's
BasicIO.open_in.)

Got the "help" function working at the interactive loop.