libsigrok/cli: Implement loglevel support.

libsigrok/cli: Implement loglevel support.

libsigrok can now be told at which loglevel to work, i.e., how many
debug/error/warning output to generate. You can also query the current
loglevel.

In sigrok-cli it is now possible to set the loglevel via -l. For example:

 - Disable all output: sigrok-cli -l 0
 - Only show errors: sigrok-cli -l 1
 - Show errors, warnings, info, and debug messages: sigrok-cli -l 4