Fix #958.

Provide a /etc/default/globaleaks.d mechanism, and do not hardfail if a default
configuration is not found.
Replace $USER with $USERNAME in globaleaks configuration file.

Following a discussion in #958, we decided to change the name of "$USER"
variable in /etc/default/globaleaks.
Merge pull request #230 from mmaker/fix/issue958

Fix #958