Don't penalize strong characters for dragging

One of the changes introduced in PR CleverRaven#37787 dropped the conditional check
for character strength being *equal* to exertion required, and instead
applied the "It takes some time" movement penalty to everyone
(regardless of strength).

And, due to the first `if` being changed to `ex >= u.get_str() + 1`,
there was no case for `ex == u.get_str()`, allowing characters with a
very specific strength, just 1 less than the strength needed, to avoid
any movement penalty at all.

Fixes CleverRaven#38104

The pain penalty is preserved (with comment), and a narrower condition
(potentially matching 2 different strength stat values) gives the "It
takes some time" message with the same movement penalty.

Any character with sufficient (or more than sufficient) strength to drag
the vehicle will see no message and receive no additional movement
penalty.

src/grab.cpp

@@ -121,12 +121,14 @@ bool game::grabbed_veh_move( const tripoint &dp )
         u.moves -= 100 * str_req / std::max( 1, u.get_str() );
         const int ex = dice( 1, 3 ) - 1 + str_req;
         if( ex > u.get_str() + 1 ) {
+            // Pain and movement penalty if exertion exceeds character strength
             add_msg( m_bad, _( "You strain yourself to move the %s!" ), grabbed_vehicle->name );
             u.moves -= 200;
             u.mod_pain( 1 );
-        } else {
-            u.moves -= 200;
+        } else if( ex >= u.get_str() ) {
+            // Movement is slow if exertion nearly equals character strength
             add_msg( _( "It takes some time to move the %s." ), grabbed_vehicle->name );
+            u.moves -= 200;
         }
     } else {
         u.moves -= 100;