feat(sys): import types from other -sys crates

[bavshin-f5]

Currently, bindgen generates bindings for everything we include: OpenSSL, PCRE2, system headers. This approach has several drawbacks:

• Lots of wasted code in bingings.rs.
• The generated bindings are lacking compared to the corresponding crates where the owners have more time on fine tuning. It's a set of small but annoying things: integer constant types, missing inline functions, functional macros etc.
• The types are not compatible: *mut nginx_sys::SSL_CTX is not the same type as *mut openssl_sys::SSL_CTX and requires unchecked pointer casts.

The code here attempts to solve this by blocklisting and replacing the external library types with types from libc, openssl-sys and pcre2-sys.

TODOs:

• The list of libc types is platform- and feature-dependent. We must check the expand_definitions output to figure out what we can and should import.
• pcre2 is too eager to attempt to build and link a static library from vendored code. Maybe we don't need that one.
• Even more platform testing.

Bonus: since we're introducing a bindgen::callbacks::ParseCallbacks implementation, we can also start adjusting the types for NGX_OK/ERROR/etc... via ParseCallbacks::int_macro().

Fixes: #99