Internal struct definitions shouldn't be exported by header files.

Instead of the trivial structure SH_Error, SH_Status is used. The name
was chosen, because error/status is set independently whether an error
has occurred. Beside the error type, it also contains the associated
errno and an error message. The error message is also printed, when it
is set. Generating error messages with variadic arguments is now also
supported.
There are also macros to check for a set status.

The exception handling was removed for the *_free methods, because they
can't fail predictably during runtime.

Unfortunately the compiler reports, that inside the macro set_status
printf may be called with NULL [printf (NULL)], although, this is
explicitly debarred.

Check in tests whether error really was written, by setting it to
undefined before each call.

Tests are done using check, allowing to integrate the tests into the
Autotools.
Methods that are part of another unit, but are called in this unit
aren't tested as this would interfere with the idea of unittests. This
also applies for purely wrapper functions, where a call is passed to
another unit.

Error handling is done by passing an Error structure as inout parameter
to functions that can fail on runtime predictable. This parameter is
always the last one. Methods, where this is not the case, doesn't have an
error parameter.
When an Error is detected, also an ERROR is passed to the log. Because
this isn't implemented yet, it is replaced by a nop.
The macro EXIT becomes now useless. It was used earlier in case of an
error, to terminate the program in the first place. This behaviour is not
userfriendly, but it can't be decided on the library's side, whether
there is an option to inform the user, something must be cleaned up or
even that recovering is possible at all.
Often these recognized errors are a non-working malloc or an over-/underflow.

Error handling can be ignored by the caller by passing NULL to the Error
parameter. Whether an error had occured, is also always possible to be
determined, by examining the return value. If the error occours in a
function returning a pointer, NULL will be returned. If it returns an value,
a special error value of that type is returned, i.e. PAGE_ERR in
SH_Data_register_page. If the return type would be void, a boolean is returned,
which tells, whether the method has succeeded.
(False means, that an error has occured.)

The error may have occured in an intern method and is passed upwards (the stack).
Internally errors are handled by an enum, but this must be considered an
implementation detail and can be changed in later versions.
It is in the responsibility of the caller to recover gracefully. It has
to be assumed that the requested operation have neither worked, nor
actually took place. Those the operation can be retried (hopefully).

Pages are hold by Data, CMS passes trough the call(s).

data handels all intern data and serves as a cache between the CMS and
the (R)DBMS.

cms will be the base "object", that performs all neccessary actions.