Extend the innodb.log_corruption test.

Remove the dependency on unzip. Instead, generate the InnoDB files
with perl.

log_block_checksum_is_ok(): Correct the error message.

recv_scan_log_recs(): Remove the duplicated error message for
log block checksum mismatch.

innobase_start_or_create_for_mysql(): If the server is in read-only
mode or if innodb_force_recovery>=3, do not try to modify the system
tablespace. (If the doublewrite buffer or the non-core system tables
do not exist, do not try to create them.)

innodb_shutdown(): Relax a debug assertion. If the system tablespace
did not contain a doublewrite buffer and if we started up in
innodb_read_only mode or with innodb_force_recovery>=3, it will not
be created.

dict_create_or_check_sys_tablespace(): Set the flag
srv_sys_tablespaces_open when the tables exist.

mysql-test/include/have_unzip.inc

@@ -1,3 +0,0 @@
-#
-# empty. the real check happens in suite.pm
-#