Explicitly specify --prefix=/usr for Python packages

Thanks to Daniel Müllner for pointing this out:
	when I updated sftpman on my Arch Linux today, I noticed that the Python
	files were installed in /home/XXX/.local/lib/... instead of /usr/lib/...

	As I found out, the reason is that I have a .pydistutils.cfg file in my
	home directory which precisely instructs distutils to install packages
	locally in my home directory. This is useful for Python packages from
	PyPi for which no AUR package exists, but of course it's not the right
	thing to do for AUR packages.

	I can of course remove my .pydistutils.cfg file and have the AUR package
	installed correctly, but I thought that other users might have similar
	issues (without noticing it maybe).

	Would you mind adding "--prefix=/usr" to the setup.py line in the
	PKGBUILD's for sftpman and sftpman-gtk