[xslt] Thread safety changes

[Jesse Pelton <jsp PKC com>]

I'm using Aleksey Sanin's XMLSec library and trying to puzzle out whether
it's thread safe. Aleksey has told me what to watch out for in his library.
He indicated that as far as he knew, libxslt was thread safe, but suggested
I check here. Since there's recently been a lively exchange about improving
thread safety, I have to assume the library is currently unsafe by default.
Is this true? If so, I have several additional questions:

- Is the unsafety limited to the generic error handler?
- Would my app be thread safe if I provide my own (thread safe) error
handler? If not, is there anything I can do to make it safe?
- Once Peter Jones' patch is applied, will libxslt be thread safe by
default?