grdvs30.c cannot use value in -C

[CovMat]

usage and code of grdvs30.c should be rewrited:

https://github.com/GenericMappingTools/gmt/blob/master/src/seis/grdvs30.c#L183-L197

now the code cannot distinguish value from filename. If you give -C1, it will see 1 as a filename

#8676

[joa-quim]

Fixed that, and other issues, in #8678.

In fact the numeric value for -C was working, just that anothrt checking routine to see if that was a file or a number printed a misleading message.