Change naming convention of POSIX emulation layer for Windows

Use prefix w32 instead of posix: Rename posixfile.c, posixio.c to
w32file.c, w32io.c.  Rename posix_accept(), posix_bind(),
posix_close(), posix_fd2socket(), posix_listen(), posix_mkdir(),
posix_setsockopt(), posix_shutdown(), posix_socket() to w32_accept(),
w32_bind(), w32_fd2socket(), w32_list(), w32_mkdir(),
w32_setsockopt(), w32_shutdown(), w32_socket().