Clean up pg_basebackup libpq usage

Clean up pg_basebackup libpq usage

When using libpq, it's generally preferrable to just use the strings
which are in the PQ structures instead of copying them out, so do
that instead in BaseBackup(), eliminating the strcpy()'s used there.

Also, in ReceiveAndUnpackTarFile(), check the string length for the
directory returned by the server for the tablespace path.