Bug 818686 - XDG Base Directory Specification support with fallback, …

…r=mt

Summary:
We check if $HOME/.pki and $HOME/.pki/nssdb exist; if they do, then we use
this path. Otherwise, use ${XDG_DATA_HOME:-$HOME/.local/share}/pki/nssdb

Test Plan:
Create dummy empty dir and set HOME to it. Then, check if getUserDb returns:
1. $HOME/.pki/nssdb when this path exists;
2. $HOME/.local/share/pki/nssdb when $HOME/.pki/nssdb does not and XDG_DATA_HOME is not defined;
3. $XDG_DATA_HOME/pki/nssdb when $HOME/.pki/nssdb does not exist and XDG_DATA_HOME is defined.

Reviewers: mt

Reviewed By: mt

Bug #: 818686

Differential Revision: https://phabricator.services.mozilla.com/D14007

--HG--
extra : rebase_source : 846de94c6fc309e56bfc9d1247f13a1b15894fbf

automation/abi-check/expected-report-libnsssysinit.so.txt

@@ -0,0 +1,8 @@
+Function symbols changes summary: 2 Removed, 0 Added function symbols not referenced by debug info
+Variable symbols changes summary: 0 Removed, 0 Added variable symbol not referenced by debug info
+
+2 Removed function symbols not referenced by debug info:
+
+  _fini
+  _init
+