Handle 5-char filenames in SlruScanDirectory

Original users of slru.c were all producing 4-digit filenames, so that
was all that that code was prepared to handle.  Changes to multixact.c
in the course of commit 0ac5ad5134f made pg_multixact/members create
5-digit filenames once a certain threshold was reached, which
SlruScanDirectory wasn't prepared to deal with; in particular,
5-digit-name files were not removed during truncation.  Change that
routine to make it aware of those files, and have it process them just
like any others.

Right now, some pg_multixact/members directories will contain a mixture
of 4-char and 5-char filenames.  A future commit is expected fix things
so that each slru.c user declares the correct maximum width for the
files it produces, to avoid such unsightly mixtures.

Noticed while investigating bug #8673 reported by Serge Negodyuck.

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