Add log_line_prefix option 'n' for Unix epoch.

Prints time as Unix epoch with milliseconds.

Tomas Vondra, reviewed by Fabien Coelho.

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

@@ -425,6 +425,7 @@
 					#   %p = process ID
 					#   %t = timestamp without milliseconds
 					#   %m = timestamp with milliseconds
+					#   %n = timestamp with milliseconds (as a Unix epoch)
 					#   %i = command tag
 					#   %e = SQL state
 					#   %c = session ID