Add a missing walIndexUnmap() call to the checkpoint code. Change a couple of SQLITE_CANTOPEN constants to SQLITE_CANTOPEN_BKPT.

FossilOrigin-Name: 1f9e8c5c27ae2c68bc01d8e2bb2c662be7b3f161
diff --git a/src/os_unix.c b/src/os_unix.c
index decf2ef..d991ec9 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -4984,13 +4984,13 @@
 
     pFile->h = open(zName, O_RDWR|O_CREAT, 0664);
     if( pFile->h<0 ){
-      rc = SQLITE_CANTOPEN;
+      rc = SQLITE_CANTOPEN_BKPT;
       goto shm_open_err;
     }
 
     rc = fstat(pFile->h, &sStat);
     if( rc ){
-      rc = SQLITE_CANTOPEN;
+      rc = SQLITE_CANTOPEN_BKPT;
       goto shm_open_err;
     }
     pFile->fid.dev = sStat.st_dev;