Add timezone offset output option to to_char()

Add ability for to_char() to output the timezone's UTC offset (OF).  We
already have the ability to return the timezone abbeviation (TZ/tz).
Per request from Andrew Dunstan

doc/src/sgml/func.sgml

@@ -5645,6 +5645,10 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
         <entry><literal>tz</literal></entry>
         <entry>lower case time-zone name</entry>
        </row>
+       <row>
+        <entry><literal>OF</literal></entry>
+        <entry>time-zone offset</entry>
+       </row>
       </tbody>
      </tgroup>
     </table>

src/backend/utils/adt/formatting.c

@@ -600,6 +600,7 @@ typedef enum
 	DCH_MS,
 	DCH_Month,
 	DCH_Mon,
+	DCH_OF,
 	DCH_P_M,
 	DCH_PM,
 	DCH_Q,
@@ -746,6 +747,7 @@ static const KeyWord DCH_keywords[] = {
 	{"MS", 2, DCH_MS, TRUE, FROM_CHAR_DATE_NONE},
 	{"Month", 5, DCH_Month, FALSE, FROM_CHAR_DATE_GREGORIAN},
 	{"Mon", 3, DCH_Mon, FALSE, FROM_CHAR_DATE_GREGORIAN},
+	{"OF", 2, DCH_OF, FALSE, FROM_CHAR_DATE_NONE},		/* O */
 	{"P.M.", 4, DCH_P_M, FALSE, FROM_CHAR_DATE_NONE},	/* P */
 	{"PM", 2, DCH_PM, FALSE, FROM_CHAR_DATE_NONE},
 	{"Q", 1, DCH_Q, TRUE, FROM_CHAR_DATE_NONE}, /* Q */
@@ -874,7 +876,7 @@ static const int DCH_index[KeyWord_INDEX_SIZE] = {
 	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
 	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
 	-1, -1, -1, -1, -1, DCH_A_D, DCH_B_C, DCH_CC, DCH_DAY, -1,
-	DCH_FX, -1, DCH_HH24, DCH_IDDD, DCH_J, -1, -1, DCH_MI, -1, -1,
+	DCH_FX, -1, DCH_HH24, DCH_IDDD, DCH_J, -1, -1, DCH_MI, -1, DCH_OF,
 	DCH_P_M, DCH_Q, DCH_RM, DCH_SSSS, DCH_TZ, DCH_US, -1, DCH_WW, -1, DCH_Y_YYY,
 	-1, -1, -1, -1, -1, -1, -1, DCH_a_d, DCH_b_c, DCH_cc,
 	DCH_day, -1, DCH_fx, -1, DCH_hh24, DCH_iddd, DCH_j, -1, -1, DCH_mi,
@@ -2502,6 +2504,16 @@ DCH_to_char(FormatNode *node, bool is_interval, TmToChar *in, char *out, Oid col
 					s += strlen(s);
 				}
 				break;
+			case DCH_OF:
+				INVALID_FOR_INTERVAL;
+				sprintf(s, "%+0*ld", S_FM(n->suffix) ? 0 : 3, tm->tm_gmtoff / SECS_PER_HOUR);
+				s += strlen(s);
+				if (tm->tm_gmtoff % SECS_PER_HOUR != 0)
+				{
+					sprintf(s, ":%02ld", (tm->tm_gmtoff % SECS_PER_HOUR) / SECS_PER_MINUTE);
+					s += strlen(s);
+				}
+				break;
 			case DCH_A_D:
 			case DCH_B_C:
 				INVALID_FOR_INTERVAL;
@@ -2915,9 +2927,10 @@ DCH_from_char(FormatNode *node, char *in, TmFromChar *out)
 				break;
 			case DCH_tz:
 			case DCH_TZ:
+			case DCH_OF:
 				ereport(ERROR,
 						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-						 errmsg("\"TZ\"/\"tz\" format patterns are not supported in to_date")));
+						 errmsg("\"TZ\"/\"tz\"/\"OF\" format patterns are not supported in to_date")));
 			case DCH_A_D:
 			case DCH_B_C:
 			case DCH_a_d: