Add comment to gzdopen() in zlib.h to use dup() when using fileno().
Add comment to gzdopen() in zlib.h to use dup() when using fileno().

A problem surfaced in a multi-threaded application where fileno() was
used to get a file descriptor from an fopen(), which was then fed to
gzdopen().  The problem occurred when the gzclose() followed by the
fclose() tried to close the same file descriptor twice.  If fclose()
were not done, there would be a memory leak.  The only way out is to
dup() the file descriptor so that gzclose() closes the duplicated
file descriptor, and fclose() closes the original file descriptor.

Streamlet/ZLibWrap,Streamlet/ZLibWrap,Streamlet/ZLibWrap,Streamlet/ZLibWrap,Streamlet/ZLibWrap