Rename no_prefix feature to unperfixed_malloc_on_supported_platforms

[SimonSapin]

As proposed in discussion starting at https://github.com/alexcrichton/jemallocator/pull/54#issuecomment-399782432.

This describes what the feature does, including its possibly-unexpected platform-specific aspect.

Avoiding negation in the name also helps argue that this feature is “correct” insofar as Cargo feature flags adding functionality and not removing them, since Cargo selects the union of features requested by multiple dependents.

[SimonSapin]

“Override malloc” is meant as implying “Override malloc and other related standard symbols such as free, etc.”

[sfackler]

We might want to have this controlled by the same feature?

--disable-zone-allocator

Disable zone allocator for Darwin. This means jemalloc won't be hooked as the default allocator on OSX/iOS.

[SimonSapin]

I don’t know what the zone allocator is. Maybe @glandium can say more on this?

[sfackler]

The zone allocator is how jemalloc takes over malloc/free on OSX. Rather than replacing the symbols themselves, it pokes at some other state during initialization.

[SimonSapin]

Then I suppose it would make sense. The question is, would there be a reason to want to control them separately?

[SimonSapin]

They are separate in jemalloc’s config, so I’ve renamed to unprefixed_malloc_on_supported_platforms in order to stick to what this does, rather than high level goals. We can add separately another flag for --disable-zone-allocator if that’s desirable.

Also added a test and some docs.

[gnzlbg]

So this LGTM. @glandium, @Amanieu, thoughts? (you might want to check https://github.com/alexcrichton/jemallocator/pull/54 and https://github.com/alexcrichton/jemallocator/pull/53 for context).

[gnzlbg]

Thanks! If this needs tunning that can be done as new issues are discovered.