std.c.openbsd: remove "msg_" prefixes from structs

Reapplies 8f14431bc883898aaf78cc985e2d90716187e882 which was reverted in
95e2605d30c15963c4d6bc9e39751031b0f52007.
04d5e07d40Andrew Kelley committed on 7/31/2023, 6:57:48 PMparent479fd2f
1 file changedLine totals unavailable