fix closing of fd's when limit is reached

when more than FD_SETSIZE file descriptors are open.
This means that if for any reason we are not able to
open more than FD_SETSIZE file descriptors then test
518 should not be run.

test 537 is all about testing libcurl functionality
when the system has nearly exhausted the number of
free file descriptors. Test 537 will try to run with
very few free file descriptors.

In this way we'll be able to sort out problems that might
arise in the prechek phase of the 518 test.

Once that 518 has been verified this change will be undone.

preventing execution of this test on many platforms

compiler warning fix since it was Ok and actually
avoids the targeted compiler warning.

Assigning the const value zero to a pointer to function
results in a null pointer value assignment to the function
pointer.

Assignment of any nonzero value is what should result in a
implementation compiler dependent result.

Since what we want to do here is the first case, this should
not trigger compiler warnings related with conversions from
'pointer to data' to 'pointer to function'.

Our autobuild test suite will judge.

and send(). Added needed HAVE_x defines.