Specify the local side of a tunnel early (FS#36711)

Needed in case the remote side of a tunnel is set to 'any'.
Better detection of hex keys

Hex WEP keys are of even length, ASCII keys are of odd length.
Using this information we make wifi-menu work better for WEP networks.
Increase default DHCP timeout

New timeout is 30 seconds, which equals the current dhcpcd default.
Allow limiting the frequencies to scan (FS#37032)

This is especially useful to enforce connecting in the 5 GHz band.
Minor rfkill refactorization

Testing whether transmission is blocked at all is now possible through

    [[ -n $(rf_status "$Interface" "$RFKill") ]]
Set a global restrictive umask

Netctl files can potentially contain passwords or execute code as root.
Add a connection type for dummy interfaces
beautify error output

Also: tag error messages as systemd error messages when the messages are
not directed to the terminal.
Overhaul of ppp connection types

- Refactorization of option file generation
- Introduction of optional additional pppd options file specification
- Introduction of unit specification option for pppoe connections
wifi-menu: ask for a name for the new profile

Some abuse of the stack, but that should be no problem: the user has to
enter around 10000 existing profile names which (s)he does not want to
overwrite before a stack overflow occurs.

Feature suggested by: Paul Bienkowski <opatutlol@aol.com>