Before this fix, gzread() would lose data if a premature end of file
was encountered. This prevented gzread() from being used on a file
that was being written concurrently. Now gzread() returns all of the
data it has available before indicating a premature end of file.
This also changes the error returned on a premature end of file from
Z_DATA_ERROR to Z_BUF_ERROR. This allows the user to determine if
the error is recoverable, which it is if Z_BUF_ERROR is returned. If
a Z_DATA_ERROR is returned, then the error is not recoverable.
This patch replaces the functionality of a previous patch that fixed
reading through an empty gzip stream in a concatenation of gzip
streams.
To implement this fix, a noticeable rewrite of gzread.c was needed.
The patch has the added advantage of using inflate's gzip processing
instead of replicating the functionality in gzread.c. This makes the
gz code a little simpler.
