Rename log_..._CONSOLE into log_channel_USERERR

log_channel_USERERR is used for error information to the user.
Used by parser or scanners for example to describe wrong input.
Translate "Unknown error" in generrtab.c

The error "unknown error" is returned for unknown error codes.
The generrtab tool now translates the text to the current language
before it is written into the err string table.
Extend errtab.text with app specific codes

The app specific error codes are set to "unknown error"
and use the value range 0x100 - 0x1ff.
Implement signalwait_t

* Implemented signalwait_t which offers an <iochannel_t> which can be waited
  on for a read event. If the iochannel_t is readable at least one signal
  from a configurable set of realtime signals was received.
* Implemented signalhandler_t which allows to set a signal handler for
  segmentation fault. initonce moved from signalconfig_t to signalhandler_t.
* Renamed rtsignal_t into signalrt_t
* Renamed signalconfig_t into signalstate_t
* Add value argument to send_signalrt. Return value given in send_rtsignal
  in wait_signalrt and trywait_signalrt.
* Implemented send2_signalrt which sends a realtime signal to a specific thread
* Added support of action "block2" to config/signalhandler which supports
  blocking a range of signals
* read_iochannel/write_iochannel are changed to not return EINTR.
  EINTR restarts read or write.
Implement skipuntilafter_utf8scanner

skipuntilafter_utf8scanner(..., char32_t uchar) skips all characters
including uchar until uchar is found.

Also renamed charmax_utf8() into maxchar_utf8().