update helpdb.jl
Simple implementation of chmod(path, mode)

currently only integer modes, can do string modes like +x later
fixes #7574

Conflicts:
	src/jl_uv.c
Allow deletion of read-only files on Windows
Fix sphinx warning for `chmod` docs.

Without this fix, the following warning appears:

```
Warning, treated as error:
/Users/vagrant/src/julia-0.3/doc/stdlib/base.rst:5388: WARNING: Inline literal start-string without end-string.
```
Change link to Unicode overview to archive.org