POSIX subsys: Need more fine-grained enable options

[pfalcon]

Currently, we have a "master switch" of CONFIG_POSIX_API which enables a lot of POSIX subsys functionality at once, and just a few sub-options to control some areas (Pthreads, etc.).

One of things which CONFIG_POSIX_API does it automatically adds headers under includes/posix/ to the include paths. Unfortunately, how they're implemented now is very fragile, and making any step further than the current tests (we don't even have POSIX samples!) go, leads to header/symbold conflicts and build errors. That's a separate matter (I'll post another ticket on that), and resolving that would likely take more time.

What I would like to do with scope of this ticket is:

1. Add separate option to control inclusion of POSIX headers.
2. Add options for things which are currently automatically enabled by CONFIG_POSIX_API: clock_gettime(), etc.
3. Then keep the current definition of CONFIG_POSIX_API, by making it select all the finer-grained options.

The purpose of these changes is to allow to gradual porting of applications. E.g., if an app needs just clock_gettime(), I should be able to pull in just that, instead of the whole POSIX subsystem, which is, again, likely will lead to header/symbol conflicts.

[pfalcon]

@nashif, @galak, @ramakrishnapallala, @nniranjhana: FYI

This comes from experience in porting https://github.com/open62541/open62541 to Zephyr (which is a Linaro project to try and see how far we're along the "take an existing 3rd-party lib and just run it with Zephyr" approach).

cc'ing you so there was a place to discuss details if needed, and these changes didn't come as a surprise (because for me some recent changes related to POSIX subsys were a surprise).