xmpp: improve C<->Lisp error handling, libstrophe logs in Lisp

- Instead of using ECL_HANDLER_CASE (which caught way too many conditions and
  broke things like the SLIME debugger), we now use ECL_RESTART_CASE to make an
  ABORT restart, and override ECL's default debugger with our own that prints
  the condition it gets and invokes this restart.
  - Note that if e.g. the SLIME debugger is running, the whole of weechat is
    halted waiting for it to complete (!)
- libstrophe logs now get handled in Lisp instead of C.
- We now make sure all of the WEE-IMPL functions are defined *before* loading
  the support library.
- We also trim blank lines from our gray stream implementation used to forward
  output to Lisp.