pgsql: In copy_file, use a palloc'd buffer instead of just a local char

Log Message:
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In copy_file, use a palloc'd buffer instead of just a local char array;
a local array isn't guaranteed to have any particular alignment, and
so it could slow down the data transfer.

Modified Files:
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pgsql/src/port:
copydir.c (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/copydir.c.diff?r1=1.12&r2=1.13)