grovel: support files with multiple dots in names

This fixes Quicklisp issue [#758](https://github.com/quicklisp/quicklisp-projects/issues/758) with [libssh2.asd](https://bitbucket.org/alxchk/cl-libssh2/src/a040f346354c65bfe218d054c668b948817fa325/libssh2.asd?at=default) that contains `libssh2-libc.cffi.lisp`, which causes error mentioned in the Quicklisp issue: [failtail.txt](http://report.quicklisp.org/cl-libssh2/2014-09-26/failtail.txt).

Without this patch `(namestring (cffi-grovel::exe-filename #P"a.b.c"))` produces an error under SBCL.