bug#69437: basename fails with suffix containing slash

[Zefram]

$ guile-3.0.9 -c '(write (basename "foo/bar" "o/bar")) (newline)'
Backtrace:
[...]
ERROR: In procedure basename:
Value out of range: 18446744073709551614

Looks like this decides based on the whole pathname that the
five-character suffix matches, and then tries to remove that suffix
from the three-character basename.  This is not sensible behaviour.
It should probably signal an error if given any suffix argument that
contains a slash character.  If not, then it should be comparing the
suffix against the basename, not the complete pathname, with the effect
that a suffix containing a slash will never be matched and so no attempt
will be made to remove it.

-zefram