The SSIZE_MAX fix didn't work on NSK, so fix it in a more-reliable

The SSIZE_MAX fix didn't work on NSK, so fix it in a more-reliable
(albeit more-complicated) way.  Problem reported by Matthew Woehlke.
* gzip.h (read_buffer): New decl.
* unlzw.c (unlzw): Use read_buffer rather than read.
* zip.c (file_read): Likewise.
* util.c (copy, fill_inbuf): Likewise.
(read_buffer, write_buffer): New functions.
(write_buf): Use write_buffer rather than write.

Undo the previous SSIZE_MAX-related change; it didn't work.
* gzip.c: Include <limits.h>.
* util.c: Likewise.
* gzip.h: Don't include <limits.h>.
(INBUFSIZ): Don't worry about SSIZE_MAX here.