RC_Channel: 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/RC_Channel/RC_Channel.cpp

@@ -31,7 +31,7 @@ const AP_Param::GroupInfo RC_Channel::var_info[] = {
     // @Param: MIN
     // @DisplayName: RC min PWM
     // @Description: RC minimum PWM pulse width. Typically 1000 is lower limit, 1500 is neutral and 2000 is upper limit.
-    // @Units: pwm
+    // @Units: PWM
     // @Range: 800 2200
     // @Increment: 1
     // @User: Advanced
@@ -40,7 +40,7 @@ const AP_Param::GroupInfo RC_Channel::var_info[] = {
     // @Param: TRIM
     // @DisplayName: RC trim PWM
     // @Description: RC trim (neutral) PWM pulse width. Typically 1000 is lower limit, 1500 is neutral and 2000 is upper limit.
-    // @Units: pwm
+    // @Units: PWM
     // @Range: 800 2200
     // @Increment: 1
     // @User: Advanced
@@ -49,7 +49,7 @@ const AP_Param::GroupInfo RC_Channel::var_info[] = {
     // @Param: MAX
     // @DisplayName: RC max PWM
     // @Description: RC maximum PWM pulse width. Typically 1000 is lower limit, 1500 is neutral and 2000 is upper limit.
-    // @Units: pwm
+    // @Units: PWM
     // @Range: 800 2200
     // @Increment: 1
     // @User: Advanced
@@ -65,7 +65,7 @@ const AP_Param::GroupInfo RC_Channel::var_info[] = {
     // @Param: DZ
     // @DisplayName: RC dead-zone
     // @Description: dead zone around trim or bottom
-    // @Units: pwm
+    // @Units: PWM
     // @Range: 0 200
     // @User: Advanced
     AP_GROUPINFO("DZ",   5, RC_Channel, dead_zone, 0),