to replace a built-in instance of InnoDB in mysqld.  This is work in
progress, with several limitations:

* Other plugins defined in the builtin InnoDB are not disabled.
  However, InnoDB in MySQL 5.1 only defines the storage engine plugin,
  no INFORMATION_SCHEMA plugins.

* The global C symbols in ha_innodb.so except innodb_hton_ptr and
  builtin_innobase_plugin will have to be redefined, e.g., by objcopy.

* The storage engine cannot be called "InnoDB" to avoid a conflict with
  the builtin name.  Here we call it InnoDBzip.

innobase_hton_name[]: Rename to "InnoDBzip" when building a dynamic plugin.

innodb_plugin_init(): New function for the dynamic plugin, to copy
and redirect configuration parameters from the builtin InnoDB.

innodb_dynamic: New configuration parameter.  This has to be added
to the builtin InnoDB in MySQL 5.1.  Also, innodb_hton_ptr must be made
global there.

innobase_init(): Interpret the parameter innodb_dynamic.

Makefile.am: Redefine class ha_innobase to ha_innodb by a preprocessor
define.  Apparently, C++ classes cannot be easily renamed by objcopy.