lib/config: canonicalize paths relative to $HOME.

Prior to 0.32, notmuch had the (undocumented) behaviour that it
expanded a relative value of database.path with respect to $HOME. In
0.32 this was special cased for database.path but broken for
database.mail_root, which causes problems for at least notmuch-new
when database.path is set to a relative path.

The change in T030-config.sh reflects a user visible, but hopefully
harmless behaviour change; the expanded form of the paths will now be
printed by notmuch config.