pginstaller - pginst: Check for existing data directory during the GUI

Log Message:
-----------
Check for existing data directory during the GUI step. This prevents rollback during execution when initdb fails on existing data directory. Instead show a dialog with the reason and suggested fixes.

Modified Files:
--------------
pginst/ca:
pginstca.c (r1.4 -> r1.5)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pginstaller/pginst/ca/pginstca.c.diff?r1=1.4&r2=1.5)