Add VSICopyFileRestartable() to allow restart of upload of large files

[rouault]

Copy a source file into a target file in a way that can (potentially)
be restarted.

This function provides the possibility of efficiently restarting upload of
large files to cloud storage that implements upload in a chunked way,
such as /vsis3/ and /vsigs/.
For other destination file systems, this function may fallback to
VSICopyFile() and not provide any smart restartable implementation.

Example of a potential workflow:

char* pszOutputPayload = NULL;
int ret = VSICopyFileRestartable(pszSource, pszTarget, NULL,
                                 &pszOutputPayload, NULL, NULL, NULL);
while( ret == 1 ) // add also a limiting counter to avoid potentiall endless looping
{
    // TODO: wait for some time

    char* pszOutputPayloadNew = NULL;
    const char* pszInputPayload = pszOutputPayload;
    ret = VSICopyFileRestartable(pszSource, pszTarget, pszInputPayload,
                                 &pszOutputPayloadNew, NULL, NULL, NULL);
    VSIFree(pszOutputPayload);
    pszOutputPayload = pszOutputPayloadNew;
}
VSIFree(pszOutputPayload);

[coveralls]

coverage: 69.095% (+0.01%) from 69.081%
when pulling a957dcc on rouault:VSICopyFileRestartable
into 4b1e7bf on OSGeo:master.