Description:
------------
snmp_set_oid_output_format only allows using the FULL (SNMP_OID_OUTPUT_FULL) or 
NUMERIC (SNMP_OID_OUTPUT_NUMERIC) setting types, neither of which is the default.  
It also has no corresponding _get_ function call to query/store and return the 
setting back to "whatever it was before I touched it"

I've attached a patch which does both (from the 5.3 branch), extends the existing 
function to include the available types in UCD Net-SNMP as of 5.4 (not 
verified/checked against older ones, have not verified that setting to _NONE will 
not cause crashes).

Test script:
---------------
$rvDefault = snmp2_get('127.0.0.1','public','.1.3.6.1.2.1.1.2.0');

snmp_set_oid_output_format(SNMP_OID_OUTPUT_FULL);
$rvFull = snmp2_get('127.0.0.1','public','.1.3.6.1.2.1.1.2.0');

snmp_set_oid_output_format(SNMP_OID_OUTPUT_NUMERIC);
$rvNumeric = snmp2_get('127.0.0.1','public','.1.3.6.1.2.1.1.2.0');

echo $rvDefault."\n";
echo $rvFull."\n";
echo $rvNumeric."\n";



Expected result:
----------------
Setting either SNMP_OID_OUTPUT_FULL or SNMP_OID_OUTPUT_NUMERIC would return the 
library to it's default.  Expect there to be an snmp_get_oid_output_format call as 
well to query the current setting.

Actual result:
--------------
Neither of the available snmp_set_oid_output_format constants can return the 
library to it's default settings.  No ability to query the library for the current 
setting.