Check for unknown flags before (rather than after) checking -o and -ohi
flags, avoiding confusing behaviour like

% ghc -bad-option -c x.hs
ghc-6.0: unrecognised flags: -bad-option

% ghc -bad-option -c x.hs -ohi x.hi
ghc-6.0: -ohi can only be used when compiling a single source file
Usage: For basic information, try the `--help' option.

% ghc -bad-option -c x.hs -o x.o
ghc-6.0: can't apply -o to multiple source files
Usage: For basic information, try the `--help' option.

(looks better with scores of flags)

please merge to STABLE