merge file_exists_in_directory and _fileExistsInDirectory functions and move into common file dumputils.c

Hi,
We have file_exists_in_directory function in pg_restore.c and same
code we are using in _fileExistsInDirectory function in pg_backup_archiver.c
also.
Here, I am attaching a patch to move these duplicate functions into
dumputils.c file

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Thanks and Regards
Mahendra Singh Thalor
EnterpriseDB: http://www.enterprisedb.com