[m-rev.] for review: rewrite rest of options_file.m

[Zoltan Somogyi]

For review by anyone.

Before commit, I intend to reorder the contents of options_file.m,
putting related predicates next to each other, but this diff leaves
predicates in their current places to make review easier.

Does anyone know whether the functionality of options_file.m has any
test cases for it, apart from Mercury.config and Mercury.options files?
I couldn't find any. I do think there should be some, but I am not sure
whether these should go in e.g. tests/hard_coded, or in a new subdirectory
of tests dedicated to tests of option file handling. (In those tests,
the code in the module(s) being compiled almost doesn't matter,
which is a pretty clear point of difference from the contents of
all the other test directories). Any preferences? And does anyone
have options files in their own projects that go beyond the usual
Mercury.{config,options} files in their complexity that I could use
as (parts of) test cases?

Zoltan.
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