Safe to add a field to svn_error_t?

[Blair Zajac]

The addition of "tracing" to svn_error_t in 1.7 currently uses a special message 
string "traced call".  Instead of doing this, can we safely add an svn_boolean_t 
to svn_error_t the indicates if it is a traced error?

While svn_error_t is public, I'm hoping most people are creating them with 
svn_error_create*(), so if that's the case, then code written and compiled 
against svn <= 1.6 would work fine when linked against a 1.7 library.

Thoughts?

Blair