Remove two hacks for some glib<2.14 versions that are no longer supported.
Drop the LDAP stuff that was never even close to finished.

There was actually a branch where LDAP support was more or less in a usable
state, but there were still some unsolved problems including the fact that
every setting has its own LDAP schema entry, which would mean lots of
maintenance overhead for pretty much every BitlBee release. :-(

Instead, we'll just stick to the XML format everywhere and just store it
in different ways.
Use xmltree to save user settings, in preparation for allowing other storage
media than local fs.
xml_load using xmltree. About half as much code as the old stuff. Likely a
little slower, but IMHO also a little less annoying to work with.
Small cleanup of xml_save().
Add xt_to_string_i() and use it to get indentation back in saved settings.
Also, use it in xt_print() instead of replicating most of xt_to_string()
in it. This changed four-space indents into tabs but oh well, we'll live.
Fixed some small leaks, restored proper read buffer size.
Make error message on using save without registering/identifying a little
more helpful.
s/ACC_SET_NOSAVE/SET_NOSAVE/.
Add xml_generate_settings(), little less code duplication.
Remove storage_rename() which was never tested/used, and current /NICK support
should be sufficient.
Removed dead bit of code that was confusing me.
Merging "storage" branch which I wrote long ago. It separates generation of
XML-formatted user configs from disk I/O so we can try to start using other
mechanisms to store them (a REST API or something, for example).
Const is hard, let's go hacking.