hostip.c: Move macOS-specific calls into global init call

hostip.c: Move macOS-specific calls into global init call

https://github.com/curl/curl/pull/7121 introduced a macOS system call
to `SCDynamicStoreCopyProxies`, which is invoked every time an IP
address needs to be resolved.

However, this system call is not thread-safe, and macOS will kill the
process if the system call is run first in a fork. To make it possible
for the parent process to call this once and prevent the crash, only
invoke this system call in the global initialization routine.

In addition, this change is beneficial because it:

1. Avoids extra macOS system calls for every IP lookup.
2. Consolidates macOS-specific initialization in a separate file.

Fixes #11252
Closes #11254