xbps-src.sh.in: improved usage().
For {binary-,}bootstrap targets clone xbps-packages repo if it's not available.
Change preference of configuration file being used.

- Don't allow root to use xbps-src, won't work due to linux-user-chroot.

- The configuration file is read as follows:

	~/.xbps-src.conf
	/etc/xbps/xbps-src.conf

  If none is found assing defaults to XBPS_DISTDIR/XBPS_MASTERDIR.

With these changes all users in a void system being part of the xbuilder group
can start building packages after running 'binary-bootstrap' or 'bootstrap' target.