Remove distribution-specific paths (and allow overrides).

This commit contains the refactoring and rewriting of code.

This commit contains the moving of files.

This commit removes an undocumented feature where IFACE_{UP,DOWN} would
be evaluated when the interface is brought up or down.
This functionality did not work as expected since the interface could
potentially be brought up/down more than once in one netcfg invocation.
This is apparent in the changes to the bonding code.

- make specifying IP optional for tuntap profiles (FS#30638)
- ethernet_{up,down} takes care of bring_interface {up,down}
- debugging updates: don't hide errors, they are useful

This is what you get when Dave Reisner points you at some bash anti-patterns.
Also in this commit:
- updated documentation
- bugfix revision of the IPv6 SLAAC address/route bug

Signed-off-by: Rémy Oudompheng <remy@archlinux.org>

Contributed-by: Byron Williams <byron@112percent.com>
Signed-off-by: Rémy Oudompheng <remy@archlinux.org>