Support configurable eventlog application names on Windows

This allows different instances to use the eventlog with different
identifiers, by setting the event_source GUC, similar to how
syslog_ident works.

Original patch by MauMau, heavily modified by Magnus Hagander

src/backend/utils/misc/postgresql.conf.sample

@@ -322,6 +322,8 @@
 #syslog_facility = 'LOCAL0'
 #syslog_ident = 'postgres'
 
+# This is only relevant when logging to eventlog (win32):
+#event_source = 'PostgreSQL'
 
 # - When to Log -