pgsql-server: Replace log_filename_prefix with more general log_filename

Log Message:
-----------
Replace log_filename_prefix with more general log_filename parameter,
to allow DBA to choose the form in which log filenames reflect the
current time.  Also allow for truncating instead of appending to
pre-existing files --- this is convenient when the log filename pattern
rewrites the same names cyclically.  Per Ed L.

Modified Files:
--------------
    pgsql-server/doc/src/sgml:
        runtime.sgml (r1.279 -> r1.280)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/runtime.sgml.diff?r1=1.279&r2=1.280)
    pgsql-server/src/backend/postmaster:
        syslogger.c (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/postmaster/syslogger.c.diff?r1=1.7&r2=1.8)
    pgsql-server/src/backend/utils/misc:
        guc.c (r1.235 -> r1.236)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/misc/guc.c.diff?r1=1.235&r2=1.236)
        postgresql.conf.sample (r1.126 -> r1.127)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.126&r2=1.127)
    pgsql-server/src/include/postmaster:
        syslogger.h (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/postmaster/syslogger.h.diff?r1=1.2&r2=1.3)