Avoid use of Z_BUF_ERROR in gz* functions except for premature EOF.

Avoid use of Z_BUF_ERROR in gz* functions except for premature EOF.

Z_BUF_ERROR was also being used for an unsuccessful gzungetc and for buffer
lengths that didn't fit in an int.  Those uses were changed to Z_DATA_ERROR
in order to assure that Z_BUF_ERROR occurs only when a premature end of
input occurs, indicating that gzclearerr() can be used.