pgsql: Add datetime macros for constants, for clarity: #define

Log Message:
-----------
Add datetime macros for constants, for clarity:

#define SECS_PER_DAY 86400
#define USECS_PER_DAY INT64CONST(86400000000)
#define USECS_PER_HOUR INT64CONST(3600000000)
#define USECS_PER_MINUTE INT64CONST(60000000)
#define USECS_PER_SEC INT64CONST(1000000)

Modified Files:
--------------
pgsql/src/backend/utils/adt:
date.c (r1.105 -> r1.106)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/date.c.diff?r1=1.105&r2=1.106)
datetime.c (r1.141 -> r1.142)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c.diff?r1=1.141&r2=1.142)
nabstime.c (r1.128 -> r1.129)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/nabstime.c.diff?r1=1.128&r2=1.129)
timestamp.c (r1.120 -> r1.121)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c.diff?r1=1.120&r2=1.121)
pgsql/src/include/utils:
timestamp.h (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/timestamp.h.diff?r1=1.40&r2=1.41)
pgsql/src/interfaces/ecpg/pgtypeslib:
dt_common.c (r1.20 -> r1.21)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/dt_common.c.diff?r1=1.20&r2=1.21)
interval.c (r1.13 -> r1.14)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/interval.c.diff?r1=1.13&r2=1.14)
timestamp.c (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/timestamp.c.diff?r1=1.23&r2=1.24)