The pthread API is considered a standard feature of libc so better to
simply merge it with the libc. Pthreads are in fact already a part of
the libc in the form of weak symbols. This merger is also a prerequisite
for better integrating pthreads with the libc I/O task.
Fix#3054
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
