Added sel4_strerror() and some convenience macros for ZF_LOG().

* Moved sel4_strerror and ZF_LOG into seL4_libs, as opposed to util_libs.
* New library function sel4_strerror():
  * Does what your gut thinks: prints a stringified version of an seL4 system
    error code.
* New macros to make logging cleaner and reduce code size:
  * Can be found in sel4_zf_logif.h.
  * ZF_LOG?_IF(condition, fmt, ...):
    * Prints to log if "condition" is true.
  * ZF_LOG?_IFERR(error, fmt, ...):
    * Prints to log if error != 0.
    * Also prints stringified version of error code.
* Added static index-checking to an array of error-code strings.
* Moved sel4_debug.[ch] from libsel4utils into libsel4debug.