Change format of backtrace to one without varibles.

This patch changes the backtrace format user when logging an error
to a format which doesn't include the variables passed to functions
in the output. This is done to avoid disclosing the password of the
user.


git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1439 44740490-163a-0410-bde0-09ae8108e29a