Add gzbuffer.3

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+.Dd November 12, 2018
+.Dt GZBUFFER 3
+.Os
+.
+.Sh NAME
+.Nm gzbuffer
+.Nd set buffer size
+.
+.Sh LIBRARY
+.Lb libz
+.
+.Sh SYNOPSIS
+.In zlib.h
+.Ft int
+.Fn gzbuffer "gzFile file" "unsigned size"
+.
+.Sh DESCRIPTION
+Set the internal buffer size
+used by this library's functions.
+The default buffer size is 8192 bytes.
+This function must be called after
+.Xr gzopen 3
+or
+.Xr gzdopen 3 ,
+and before any other calls
+that read or write the file.
+The buffer memory allocation
+is always deferred to the first read or write.
+Three times that size in buffer space is allocated.
+A larger buffer size of,
+for example,
+64K or 128K bytes
+will noticeably increase the speed
+of decompression (reading).
+.
+.Pp
+The new buffer size also affects
+the maximum length for
+.Xr gzprintf 3 .
+.
+.Sh RETURN VALUES
+.Fn gzbuffer
+returns 0 on success,
+or -1 on failure,
+such as being called too late.