Add POSIX confstr API

[jgarzik]

Fixes #3767

Some notes for reviewers:

• This PR was successfully tested with my Rust getconf util implementation at /~https://github.com/rustcoreutils/posixutils-rs
• It would be ideal, and seemingly appropriate, to add the confstr(3) API unix, because it is POSIX standard, just like sysconf(3), and originates from libc, just like sysconf(3). This failed to due a single platform, arm-android.
• Therefore, an ideal future patch would populate other unix platforms with confstr.

This patch chooses the more limited path of focusing on linux-gnu, given that the symbols (not the function!) are glibc-specific.

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[jgarzik]

Status: all tests except arm-linux-androidabi are passing. Unclear what is the best solution around this test failure.

[devnexen]

Missing functions are either obsolete or explicitly disallowed by SELinux:

a64l/l64a
confstr
crypt/encrypt/setkey
gethostid
shm_open/shm_unlink
sockatmark
ualarm

source

[jgarzik]

Status: Narrowed to strictly gnu-linux, as the updated OP describes more fully.

Ready for merging, with an aspiration to see wider confstr beyond just linux-gnu and apple.

[devnexen]

fair enough, even tough I believe musl for example has it too for example but that s fair as is :)

[jgarzik]

fair enough, even tough I believe musl for example has it too for example but that s fair as is :)

The Platonic ideal would seem to be "confstr on all unix except x, y, and z" but that is difficult to express in the semver tests, I'm guessing.

Further complicating the task is that value of _CS_PATH is not specified and may vary across confstr-supporting OS's. confstr is a POSIX-wide API with local-libc-specific values and symbology (thus the difference in Darwin and glibc symbols CS*)