Defining a global EFFECTIVE_MAKE_JOBS, using it for Rust packages, MAKE_JOBS_SAFE=no on rust packages for netbsd

[coypu%sdf.org@localhost]

Hi folks,

With the intention of making cargo respect MAKE_JOBS (it defaults to
number of CPUs), I created this diff.

Please post your thoughts. I'd like to have fewer bulkbuild failures and
frustrated-of-Rust users. I am not at all opinionated on the method of
achieving this.

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Patch 1:
Add a visible EFFECTIVE_MAKE_JOBS, a tunable resembling _MAKE_JOBS,
but is defined as 1 in the case of no jobs.

No -j prefix for maximum usability.
(XXX question: is _DEF_VARS the right place to add it?)

http://coypu.sdf.org/cargo.part1.diff

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Patch 2:
Make many cargo-using packages respect MAKE_JOBS

http://coypu.sdf.org/cargo.part2.diff

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Patch 3:
Mark cargo-using packages as MAKE_JOBS_SAFE=no on netbsd, to workaround
an rtld issue.

http://coypu.sdf.org/cargo.part3.diff

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Previous two versions:
1. Use a CARGO_MAKE_JOBS which is 1 if MAKE_JOBS_SAFE=no
(jperkin prefers fewer THING_MAKE_JOBS, to avoid duplication)

2. Use _MAKE_JOBS. This variable is -jTHING and empty if
MAKE_JOBS_SAFE=no.
I was going to switch tit to -j1 if MAKE_JOBS_SAFE=no, but joerg thoguht
this is risky.