client: Add support for tethering

Add tethering support. For WiFi also set the SSID and passphrase if they
have been specified on the command line. Check that setting both SSID and
passphrase succeed before trying to enable tethering.

After successfully setting SSID and passphrase for WiFi, use the boolean
value parsing helper function return value to decide whether WiFi tethering
is to be enabled, disabled or not modified.

Fixes BMC#25886
client: Follow coding style

Follow coding style and use __connmanctl prefix for the commands() function.
client: Add command completion

Add completion support for connmanctl commands.
client: Trim off spaces from user input

After splitting the input string into components, create a new array holding
the non-zero substrings as input for the commands. Thus the input can start
and end as well as divide its separate parts by any number spaces.