pgsql: Restructure log_line_prefix options to be clearer:

Log Message:
-----------
Restructure log_line_prefix options to be clearer:

#log_line_prefix = '' # %u = user name
# %d = database name
# %r = remote host and port
# %h = remote host
# %p = PID
# %t = timestamp
# %m = timestamp with milliseconds
# %i = command tag
# %c = session id
# %l = session line number
# %s = session start timestamp
# %x = transaction id
# %q = stop here in non-session processes
# %% = '%'
# e.g. '<%u%%%d> '

Modified Files:
--------------
pgsql/src/backend/utils/misc:
postgresql.conf.sample (r1.144 -> r1.145)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.144&r2=1.145)