Add local-address escape "%L" to log_line_prefix.

This escape shows the numeric server IP address that the client
has connected to.  Unix-socket connections will show "[local]".
Non-client processes (e.g. background processes) will show "[none]".

We expect that this option will be of interest to only a fairly
small number of users.  Therefore the implementation is optimized
for the case where it's not used (that is, we don't do the string
conversion until we have to), and we've not added the field to
csvlog or jsonlog formats.

Author: Greg Sabino Mullane <htamfids@gmail.com>
Reviewed-by: Cary Huang <cary.huang@highgo.ca>
Reviewed-by: David Steele <david@pgmasters.net>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAKAnmmK-U+UicE-qbNU23K--Q5XTLdM6bj+gbkZBZkjyjrd3Ow@mail.gmail.com

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

@@ -604,6 +604,7 @@
 					#   %d = database name
 					#   %r = remote host and port
 					#   %h = remote host
+					#   %L = local address
 					#   %b = backend type
 					#   %p = process ID
 					#   %P = process ID of parallel group leader