wpa: add Frequency= option

This option sets a frequency to use in ad-hoc mode for a newly created IBSS
(when it is going to be created).
Minor fixes

- Don't bring an interface down if it is not brought up for wireless
connections.

- Often, `read` needs '-r'.
Fix default interface bindings (fourth time)

The following changes introduced in Bash 4.3 necessitate this commit:
- Setting the array attribute no longer initializes an array.
- Empty-but-set arrays no longer pass [[ -v ]].

Previous attempts:
feb669f4b2a15e6ea1d84be336f12be4650f9d2d
711c46457ae9fef52c7c529d89c67d0d526f73ef
e8210827fd59e4539a70af55cb452b211633fedd
Fix pid lookup for killing pppd

The pid file contains more than just the pid: on the second line it
records the interface pppd manages.
Add is-enabled subcommand to netctl
1.7 release updates