[m-dev.] vim modelines in Mercury.options files

[Zoltan Somogyi]

At the moment, many don't have one, but the ones that do are inconsistent:

tests/invalid/Mercury.options:# vim: ts=4 sw=4 expandtab
tests/invalid_make_int/Mercury.options:# vim: ts=8 sw=8 noexpandtab
tests/invalid_nodepend/Mercury.options:# vim: ts=4 sw=4 expandtab
tests/invalid_submodules/Mercury.options:# vim: ts=4 sw=4 expandtab
tests/valid/Mercury.options:# vim: ts=8 sw=8 noexpandtab
tests/valid_make_int/Mercury.options:# vim: ts=8 sw=8 noexpandtab
tests/valid_seq/Mercury.options:# vim: ts=8 sw=8 noexpandtab
tests/warnings/Mercury.options:# vim: ts=8 sw=8 noexpandtab

Some require spaces and forbid tabs, some allow tabs and discourage spaces.
I think we should pick one. I have a slight preference for ts=4 sw=4 expandtab,
as this is more flexible in fitting option settings on one line,
but uniformity is more important than the specific choice.

Opinions?

Zoltan.