nrf_modem_lib: don't require POSIX sockets names

[bandogora]

• Replace POSIX API macros with Zephyr socket API (ZSOCK) macros in sanity.c
• Don't imply NET_SOCKETS_POSIX_NAMES if !POSIX_API

CONFIG_POSIX_API enables experimental symbols and the NET_SOCKETS_POSIX_NAMES symbol is deprecated. POSIX names don't seems to be required for the library with the exception of 8 BUILD_ASSERTs. All of those BUILD_ASSERTs use POSIX macros that are aliases for ZSOCK macros.

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[rlubos]

@bandogora There are more changes needed in libraries/samples related to removing NET_SOCKETS_POSIX_NAMES , however good news is all this is already being addressed in the upmerge PR: #19720

[bandogora]

is all this is already being addressed in the upmerge PR: #19720

Sounds good!