config: only exclude messages if 'search.exclude_tags' is explicitly set

Currently, the 'search.exclude_tags' option is automatically
set to "deleted;spam;" if it's missing from the config file.

This violates the Principle of Least Surprise, so *only* set
'search.exclude_tags' to "deleted;spam;" if we didn't find a
configuration file at all.

This patch is actually Austin Clements' work:
  id:"20120117203211.GQ16740@mit.edu"