Rename LwIP library in preparation for removal of LwIP libc plugin. The
current LwIP library will be replaced with a new version stripped of its
synchronous socket support. The next version will be incompatible with
the current, so removing 'lwip.lib.so' completely for a period makes it
easy to identify legacy users.
Fix#2797
By building the posix library as shared object with an ABI, we
effectively decouple posix-using programs from the library
implementation (which happens to depend on several os-level APIs such as
the VFS).
