Fix for log_line_prefix and session display

Currently, our session id, displayed by log_line_prefix and CSV output,
is made up of the session start time epoch seconds and the process id.
The problem is that the printf mask is currently %lx.%x, causing a
process id less than 4096 to not display a full four hex digits after
the decimal point.  I think this is confusing because the number .423
appears higher than .1423, though it is not.  Here is what our current
output looks like with log_line_prefix="%c: ":

    50785b3e.7ff9: ERROR:  syntax error at or near "test" at character 1
    50785b3e.7ff9: STATEMENT:  test
    50785b3e.144: ERROR:  syntax error at or near "test" at character 1
    50785b3e.144: STATEMENT:  test

With my fix, here is the updated output:

    507864d3.7ff2: ERROR:  syntax error at or near "test" at character 1
    507864d3.7ff2: STATEMENT:  test
    507864d3.013d: ERROR:  syntax error at or near "test" at character 1
    507864d3.013d: STATEMENT:  test

Patch attached.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +