The core no longer needs to run the event polling loop as fetches are now
scheduler driven. This is part of a series which will ultimately remove
netsurf_poll callback altogether.
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.
