More fixes as per the reviews.

- Took out ${prefix}-
- get_var sanity
- pt-mysql-summary now follows pt-summary's suit and doesn't take
  in files to write to
- pt-summary defers setting the CMD_FOO vars to a function, as
  otherwise it would skip our PATH mungling.

t/pt-mysql-summary/samples/expected_output_temp002.txt

@@ -173,6 +173,10 @@ Uptime                                    90000           1           1
   performance_schema                                                            
 
 # Noteworthy Technologies ####################################
+       Full Text Indexing | No
+         Geospatial Types | No
+             Foreign Keys | No
+             Partitioning | No
                       SSL | No
      Explicit LOCK TABLES | No
            Delayed Insert | No