[RFC] Add dynamic runtime verbosity selection

[TobleMiner]

depends on #20 (first commit is from #20)

This commit adds dynamic verbosity selection to ddhcpd. It introduces a new command line option -V which increases the log level. The compile time option LOG_LEVEL has been removed and replaced by two new options:

• LOG_LEVEL_LIMIT Sets the maximum debug level available at runtime. Can be used to reduce memory footprint slightly, cuts out all unused logging code by replacing corresponding log macros by empty macros

• LOG_LEVEL_DEFAULT Sets the default log level

The current log level can be queried and set through the global variable log_level.

Additionally this adds the option -Vto ddhcpdctl, too. It can be used to set the log level of ddhcpd on the fly.

Though this might be a good idea especially since we might want to debug ddhcpd on devices while they are in normal use without getting continous log spam.

[TobleMiner]

I've just updated this PR to fix the following issues:

• In quite a few places LOG_LEVEL was not replaced by LOG_LEVEL_LIMIT
• Log levels were defined through an enum. That didn't work since a lot of debug code is supposed to be eliminated by the preprocessor already