pcbnew: adjust default soldermask/dam clearance

The previous clearance values were equivalent to the default copper
clearance, which could cause issues for users who didn't adjust the
values and had alignment errors at the board house.  New values default
to typical dry film minimums.

( cherry-picked from 2c5154c63 )

Fixes: lp:1786765
* https://bugs.launchpad.net/kicad/+bug/1786765

include/board_design_settings.h

@@ -49,8 +49,8 @@
 
 #define DEFAULT_PCB_EDGE_THICKNESS    0.15
 
-#define DEFAULT_SOLDERMASK_CLEARANCE  0.2
-#define DEFAULT_SOLDERMASK_MIN_WIDTH  0.0
+#define DEFAULT_SOLDERMASK_CLEARANCE  0.051  // soldermask to pad clearance
+#define DEFAULT_SOLDERMASK_MIN_WIDTH  0.25   // soldermask minimum dam size
 #define DEFAULT_SOLDERPASTE_CLEARANCE 0.0
 #define DEFAULT_SOLDERPASTE_RATIO     0.0