pgsql: Code review for pg_verify_checksums.c.

Code review for pg_verify_checksums.c.

Use postgres_fe.h, since this is frontend code.  Pretend that we've heard
of project style guidelines for, eg, #include order.  Use BlockNumber not
int arithmetic for block numbers, to avoid misbehavior with relations
exceeding 2^31 blocks.  Avoid an unnecessary strict-aliasing warning
(per report from Michael Banck).  Const-ify assorted stuff.  Avoid
scribbling on the output of readdir() -- perhaps that's safe in practice,
but POSIX forbids it, and this code has so far earned exactly zero
credibility portability-wise.  Editorialize on an ambiguously-worded
message.

I did not touch the problem of the "buf" local variable being possibly
insufficiently aligned; that's not specific to this code, and seems like
it should be fixed as part of a different, larger patch.

Discussion: https://postgr.es/m/1535618100.1286.3.camel@credativ.de

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d787af7badfe8c4c9afb7f0af9958a69b723a212

Modified Files
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src/bin/pg_verify_checksums/pg_verify_checksums.c | 47 +++++++++++------------
1 file changed, 23 insertions(+), 24 deletions(-)