Debian package: Support XULRunner up to 22

Support for newer Xulrunners closes #699239

Also bump package version to today
Debian package: Bump Standards-Version to 3.9.4
Debian package: Upload as 1.0~~pre+git130407-1 to Debian Experimental
load_rc: avoid NS_ERROR_FILE_TARGET_DOES_NOT_EXIST in symlink test

Previously, if there was no rc, the symlink test on the rc path would
throw NS_ERROR_FILE_TARGET_DOES_NOT_EXIST.  This patch introduces a catch
for that error.

file_symlink_p: new util.  takes an nsIFile and returns the value of
file.isSymlink(), but also catches the error and returns false if the file
does not exist.  Note that this cannot be tested with file.exists(),
because that method automatically resolves symlinks.