AP_AdvancedFailsafe: Use SI units conventions in parameter units

Follow the rules from:
http://physics.nist.gov/cuu/Units/units.html
http://physics.nist.gov/cuu/Units/outside.html
and
http://physics.nist.gov/cuu/Units/checklist.html
one further constrain is that only printable (7bit) ASCII characters are allowed

libraries/AP_AdvancedFailsafe/AP_AdvancedFailsafe.cpp

@@ -81,20 +81,20 @@ const AP_Param::GroupInfo AP_AdvancedFailsafe::var_info[] = {
     // @DisplayName: AMSL limit
     // @Description: This sets the AMSL (above mean sea level) altitude limit. If the pressure altitude determined by QNH exceeds this limit then flight termination will be forced. Note that this limit is in meters, whereas pressure altitude limits are often quoted in feet. A value of zero disables the pressure altitude limit.
     // @User: Advanced
-    // @Units: meters
+    // @Units: m
     AP_GROUPINFO("AMSL_LIMIT",   8, AP_AdvancedFailsafe, _amsl_limit,    0),
 
     // @Param: AMSL_ERR_GPS
     // @DisplayName: Error margin for GPS based AMSL limit
     // @Description: This sets margin for error in GPS derived altitude limit. This error margin is only used if the barometer has failed. If the barometer fails then the GPS will be used to enforce the AMSL_LIMIT, but this margin will be subtracted from the AMSL_LIMIT first, to ensure that even with the given amount of GPS altitude error the pressure altitude is not breached. OBC users should set this to comply with their D2 safety case. A value of -1 will mean that barometer failure will lead to immediate termination.
     // @User: Advanced
-    // @Units: meters
+    // @Units: m
     AP_GROUPINFO("AMSL_ERR_GPS", 9, AP_AdvancedFailsafe, _amsl_margin_gps,  -1),
 
     // @Param: QNH_PRESSURE
     // @DisplayName: QNH pressure
     // @Description: This sets the QNH pressure in millibars to be used for pressure altitude in the altitude limit. A value of zero disables the altitude limit.
-    // @Units: millibar
+    // @Units: mbar
     // @User: Advanced
     AP_GROUPINFO("QNH_PRESSURE", 10, AP_AdvancedFailsafe, _qnh_pressure,    0),
 
@@ -142,7 +142,7 @@ const AP_Param::GroupInfo AP_AdvancedFailsafe::var_info[] = {
     // @DisplayName: RC failure time
     // @Description: This is the time in seconds in manual mode that failsafe termination will activate if RC input is lost. For the OBC rules this should be (1.5). Use 0 to disable.
     // @User: Advanced
-    // @Units: seconds
+    // @Units: s
     AP_GROUPINFO("RC_FAIL_TIME",   19, AP_AdvancedFailsafe, _rc_fail_time_seconds,    0),
 
     AP_GROUPEND