posix: implement sysconf()

[cfriedt]

Is your enhancement proposal related to a problem? Please describe.
The sysconf() utility is actually quite fantastic to have available for any POSIX environment. For the most part, it is used to have a portable way to query standard system limits that do not change after initialization (including those that are compile-time constant).

Describe the solution you'd like
An implementaiton of sysconf(), possibly with a space-optimized version

This is one of those interesting cases where you could provide a "small, medium, and large" configuration, cc @stephanosio.

XS: O(1) time, O(1) space
Completed entirely via macros. Not POSIX-ly correct. Would be compile-time const only. Likely would not even compile if an invalid argument is provided, which would be kind of desireable. Would not work for dynamic queries - e.g. int n; sysconf(n).

// <unistd.h> or somewhere else more appropriate
// ...
#define __z_posix_sysconf_SC_CLK_TCK CONFIG_SYS_CLOCK_TICKS_PER_SEC
#define __z_posix_sysconf_SC_AIO_MAX (-1) // not supported (could be guarded by some Kconfig)
// ...
#define sysconf(x) __z_posix_sysconf ## x

// application
__ASSERT_NO_MSG(sysconf(_SC_CLK_TCK) == CONFIG_SYS_CLOCK_TICKS_PER_SEC);
__ASSERT_NO_MSG(sysconf(_SC_AIO_MAX) == -1);
__ASSERT_NO_MSG(sysconf(42) == 73); // nonsense - will trigger a compile error

Basically free and carefree. This should probably be the default.

S: O(n) time, O(n) space
Linear search through the n enabled options, where it's possible that n < k, where k is the minimum number of supported sysconf keys. Not POSIX-ly correct. Not able to distinguish between an invalid value and a value that is not supported.

M: O(1) time, O(m) space
LUT made of the m maximum sysconf options that Zephyr supports, where m >= k, the minimum number required to be POSIX-ly correct. This method is able to distinguish between invalid and unsupported arguments.

While this is constant is size, it is spatially unstable if the key-space is sparse. Specifically, that would be the case when the libc did not use a contiguous key space (starting at 0, ideally).

It would probably be desireable in this case to have a BUILD_ASSERT() that checks the size of the table (implies the key-space is contiguous).

To give an idea of the space requirements, there are currently 125 required sysconf keys. On 64-bit systems, a long is 8 bytes, so this would be 1000 bytes alone.

L: O(log(m)) time, O(m) space
A binary search through the m maximum sysconf options that Zephyr supports. Same definition as above for k. This method is able to distinguish between invalid and unsupported arguments. It supports a sparse set of keys.

The bsearch() standard library call can be used here. If keys cannot be sorted at build time in a compile-time const way, then there can be a SYS_INIT() call that reorders them in RAM, at the expense of RAM.

Describe alternatives you've considered
Not implementing the sysconf POSIX feature.

Additional context
https://pubs.opengroup.org/onlinepubs/9699919799/functions/sysconf.html
https://man7.org/linux/man-pages/man3/sysconf.3.html

[cfriedt]

There are many reasons to use sysconf() in POSIX applications. In particular, simply supporting compilation of 3rd party portable libraries.

However, even several internal POSIX function implementations could use it. E.g. the times() function, requested in #51978.