`mkdir -p' would create parent directories with permissions

`mkdir -p' would create parent directories with permissions
that did not account for the umask. [introduced with the
2000-09-30 change that became part of fileutils-4.0.28]

Include dirname.h.
Compute the parent directory `mode' unconditionally, effectively
as `$(umask -S),u+wx'.
Use make_path to create only the parent directories, thus using
the same code, both with and without -p, to create the final
component in each file name.  Reported by Bob Proulx.