behaviour some versions of IRIX exhibit of committing suicide on big
mallocs instead of just returning a friendly null pointer

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.