Fix typo in configure

Fixed typo in AC_MSG_ERROR for xattr.
core: Add abort in case of overread from client
libbuxton: Fix bug in hashmap freeing that can cause a segfault

When the client connection is closed, the hashmap is freed, but the
pointer to it was not set to null. Then when a new connection is made,
the key_hash pointer was checked if it points to null before trying
to access the hashmap. This caused a segfault when trying to
dereference the function pointer for the hash function, so it's now
being set to NULL to be able to test whether it was freed.
Release buxton v3
Simplifying public Buxton API

BuxtonSimple is an API that allows users who are new to Buxton or
familiar with vconf to utilize Buxton. There are fewer function
arguments for each function.  This is possibile because there are
default call back functions and a global client provided in
the library. There are also newer functions similar to those found
in the vconf API (such has set_int32). The opening and closing of the
client connection is embedded into each function called so the programmer
does not have to call buxton_open() or buxton_close().

There is a library called libbuxtonsimple that can be linked to by a client
program when using BuxtonSimple (-lbuxtonsimple). This exposes only the
funcitons in the BuxtonSimple API, without the functions from the original
Buxton API.

Man pages for the BuxtonSimple API and all public functinos are included as
well as demo code.

Test units are also included in the /test directory.
Changed README and man pages to indicate that buxtonsimple is synchronous