Allow minimal unix target to build in cygwin (but not necessarily run).

Allow minimal unix target to build in cygwin (but not necessarily run).

Applied a patch from Marc Boucher which has the following changes:

1) Fixed sys_arch.txt documentation to have new return type from
   sys_thread_new.

2) Removed unnecessary casts on calling sys_timeout() in certain files.

3) Removed some unnecessary break statements after return statements.

4) Changed sys_timeout_remove() to sys_untimeout().

5) Added some forgotten #ifndef SYS_LIGHTWEIGHT_PROT to memp.c

6) Changed LWIP_DIAG and LWIP_PLATFORM_ASSERT to have do while().