AP_Parachute: 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_Parachute/AP_Parachute.cpp

@@ -28,7 +28,7 @@ const AP_Param::GroupInfo AP_Parachute::var_info[] = {
     // @DisplayName: Parachute Servo ON PWM value
     // @Description: Parachute Servo PWM value when parachute is released
     // @Range: 1000 2000
-    // @Units: pwm
+    // @Units: PWM
     // @Increment: 1
     // @User: Standard
     AP_GROUPINFO("SERVO_ON", 2, AP_Parachute, _servo_on_pwm, AP_PARACHUTE_SERVO_ON_PWM_DEFAULT),
@@ -37,7 +37,7 @@ const AP_Param::GroupInfo AP_Parachute::var_info[] = {
     // @DisplayName: Servo OFF PWM value
     // @Description: Parachute Servo PWM value when parachute is not released
     // @Range: 1000 2000
-    // @Units: pwm
+    // @Units: PWM
     // @Increment: 1
     // @User: Standard
     AP_GROUPINFO("SERVO_OFF", 3, AP_Parachute, _servo_off_pwm, AP_PARACHUTE_SERVO_OFF_PWM_DEFAULT),
@@ -46,7 +46,7 @@ const AP_Param::GroupInfo AP_Parachute::var_info[] = {
     // @DisplayName: Parachute min altitude in meters above home
     // @Description: Parachute min altitude above home.  Parachute will not be released below this altitude.  0 to disable alt check.
     // @Range: 0 32000
-    // @Units: Meters
+    // @Units: m
     // @Increment: 1
     // @User: Standard
     AP_GROUPINFO("ALT_MIN", 4, AP_Parachute, _alt_min, AP_PARACHUTE_ALT_MIN_DEFAULT),
@@ -55,7 +55,7 @@ const AP_Param::GroupInfo AP_Parachute::var_info[] = {
     // @DisplayName: Parachute release delay
     // @Description: Delay in millseconds between motor stop and chute release
     // @Range: 0 5000
-    // @Units: Milliseconds
+    // @Units: ms
     // @Increment: 1
     // @User: Standard
     AP_GROUPINFO("DELAY_MS", 5, AP_Parachute, _delay_ms, AP_PARACHUTE_RELEASE_DELAY_MS),