etcupdate: preserve the metadata of the destination file

[vangyzen]

Summary

When using diff3 to perform a three-way merge, etcupdate lost the destination
file's metadata. The metadata from the temporary file were used instead.
This was unpleasant for rc.d scripts, which require execute permission.
Use "cat >" to overwrite the destination file's contents while preserving its
metadata.

Test Plan

All kyua tests still pass, although they don't test this case.

head/usr.sbin/etcupdate/etcupdate.sh

#ifdef INET6
         case INC_ISIPV6:
                 SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
                 SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
                 break;
 #endif
         }
         SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
         SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
+        SipHash_Update(&ctx, &irs, sizeof(irs));
         SipHash_Update(&ctx, &flags, sizeof(flags));
         SipHash_Update(&ctx, &secmod, sizeof(secmod));
         SipHash_Final((u_int8_t *)&siphash, &ctx);
 
         return (siphash[0] ^ siphash[1]);
 }
 
 static tcp_seq