This removes the toskip parameter, which was only used by the RISC OS
front end. The toskip param was used to skip 8 characters which did
not need to be escaped from the start of the URL. The RISC OS front
end now orders the steps of its URL construction to avoid the need
for this.
The avoids situations were we threw away the length, only for
the caller to have to strlen the returned string.
Note, there seems to be a case of the amiga front end writing
beyond end of allocation. Added a TODO for now.
This rationalises the path construction and basename file
operations. The default implementation is POSIX which works for all
frontends except windows, riscos and amiga which have differeing path
separators and rules.
These implementations are significantly more robust than the previous
nine implementations and also do not use unsafe strncpy or buffers
with arbitrary length limits.
These implementations also carry full documentation comments.
