If NSG_CLOSE_IMMEDIATELY is on, then the socket will be closed,
which means that (if the socket was added to either read_set or
write_set) the subsequent select will in turn be performed on a
socket that has been closed.

Standard socket implementations seem to ignore a descriptor
referring to a closed socket but LWIP
(http://savannah.nongnu.org/projects/lwip/) crashes. So better
not to add to the sets.

MONGOOSE_NO_CGI is implied by both NS_DISABLE_SOCKETPAIR and
MONGOOSE_NO_FILESYSTEM. Thus, so as not to get complaints about
the same define being defined twice, some extra checks are needed.

On very resource constraind devices you need to be able to
control the size of all buffers and structures placed on the
stack.

Low-end systems, not running a proper OS, may not have the
concept of a user.

Some unix environments defines O_BINARY.

If you do not have a filesystem or do not support dav or
dynamic loading, the corresponding header files need not
be included (some environments might not have them).