The "verbose" alternative for assert() is now an expression and
can be used in all the places that a funcion call can be used.

g++ gives the error message "error: invalid conversion from ‘void*’ to
‘list_node*’ [-fpermissive]"

I applied the following terminology and changed the wording in the doc
accordingly:

* must not: If the parameter is of the value it *must not* be it either
  hits an assert or crashes the system.
* may not: The value can be that value, but the function will return an
  error.

Updated file comment header to comply with coding standard;
added brief module description and reference to tsrb to guide reader
who might be looking for thread-safe implementation

and improve docs for THREAD_FLAG_TIMEOUT

Everything below the thread states was cut off in the module
documentation because Doxygen does not handle nested @name tags.

Fix xtimer_now() usage and fix columns alignment in ps command when
module schedstatistics is used.

cppcheck said: core/assert.c:23: warning (invalidPrintfArgType_sint): %d in
format string (no. 2) requires 'int' but the argument type is 'unsigned
int'.