Rename register_unlink_segment() to register_unlink_tombstone()

Rename register_unlink_segment() to register_unlink_tombstone()

Only "tombstone" files (first segment of main fork) are unlinked after
checkpoints, so rename the function and remove the extra arguments to
make that more clear.

Additionally, add an assertion in mdunlinkfiletag() that the FileTag
only contains expected values.

Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAEze2WjfP95SL_Hsu7GzYXLnQyEsT49zOnNvbY_mBLCFiQra1g@mail.gmail.com