Fix gzeof() to behave just like feof() when read is not past end of file.

Before, gzeof() would return true (accurately) when the last read request
went just up to the end of the uncompressed data.  In the analogous case,
feof() would return false, only returning true when a read request goes
past the end of the file.  This patch corrects gzeof() to behave in the
same way as feof(), as noted in the zlib.h documentation.

gzguts.h

@@ -155,9 +155,10 @@ typedef struct {
     unsigned char *out;     /* output buffer (double-sized when reading) */
     int direct;             /* 0 if processing gzip, 1 if transparent */
         /* just for reading */
-    int eof;                /* true if end of input file reached */
-    z_off64_t start;        /* where the gzip data started, for rewinding */
     int how;                /* 0: get header, 1: copy, 2: decompress */
+    z_off64_t start;        /* where the gzip data started, for rewinding */
+    int eof;                /* true if end of input file reached */
+    int past;               /* true if read requested past end */
         /* just for writing */
     int level;              /* compression level */
     int strategy;           /* compression strategy */