socketpool, ssl: rationalize return types & throwing exceptions

[jepler]

While looking into #7606 @dhalbert noted that there were irregularities in our socket-like APIs that may have contributed to the bug existing in the first place.

We should consider rationalizing the code so that:

• the common-hal APIs consistently return either positive counts or negative errno values, in signed types (mp_int_t)
• the shared-bindings have the responsibility of transforming negative errno values into OSError exceptions
• noting, at least in comments, common-hal APIs that are not permitted to assume the VM is running (no alloc, no exception)

[tannewt]

I (inconsistently) drop the common_hal_ prefix on functions meant to be used outside of the VM.

[gneverov]

Without the common_hal_ prefix the functions can sometimes collide with functions in 'shared-bindings' which already typically don't have a prefix.

[tannewt]

Yup! I usually add a _ prefix which I use for statics ala Python's private designation.

[dhalbert]

Maybe we could use a base_ prefix or similar for the functions that can be called from C outside the VM. Maybe you can think of a better prefix. I too usually drop the common_hal_ for routines not meant to be called via a Python API.