client, sysdb: Improved error reporting.
configure: Report an error when encountering invalid settings.
configure, src/Makefile: Sort plugins alphabetically.
core/data: Make string and binary data not constant.

This will allow for more flexibility. However, it also requires to create a
copy of data queried using libdbi.
dbi_test: Mocked required dbi*_copy_idx() functions.
data: Added helper functions to copy and free data.
utils dbi: Use sdb_data_free_datum().
store: Added support for different data-types for attributes.

An attribute value is now stored as sdb_data_t, supporting all data-types
which are supported by that type.

JSON-serialization does not fully support that yet, though.
core/data.h: Removed obsolete comment.
core/data: Added sdb_data_format() function.

This function creates a string representation of a datum using a default
format.
Merged branch 'master' of git://git.tokkee.org/sysdb.
sysdb: Split sdb_command_exec() into two functions.

Separate the handling of the server's reply to make it reusable by other parts
of the code.
sysdb: Use the asynchronous readline interface; handle asynch server replies.

For this, setup the terminal in non-canonical, raw mode in order to be able to
handle each single character input. This allows to handle user input and
asynchronous server replies at the same time but without the need to give up
sequential operation (no threads are required for the asynchronous operation).
Instead, select() is used to do the input multiplexing allowing to easily
handle different output.