Rename snapmgmt.c/h to snapmgr.c/h, for consistency with other files.

src/backend/access/heap/heapam.c

src/backend/access/index/indexam.c

src/backend/access/nbtree/nbtpage.c

src/backend/access/transam/subtrans.c

src/backend/access/transam/transam.c

src/backend/access/transam/xact.c

src/backend/catalog/heap.c

src/backend/catalog/index.c

src/backend/commands/cluster.c

src/backend/commands/copy.c

src/backend/commands/explain.c

src/backend/commands/indexcmds.c

src/backend/commands/portalcmds.c

src/backend/commands/prepare.c

src/backend/commands/tablecmds.c

src/backend/commands/trigger.c

src/backend/commands/vacuum.c

src/backend/commands/variable.c

src/backend/executor/functions.c

src/backend/executor/nodeBitmapHeapscan.c