clib2 Code

- <stdarg.h> now tries to use the compiler supplied, machine specific
<stdarg.h> file and also includes the OS4-specifc <sys/amigaos-va.h>
file, if necessary.

- Added another test to <sys/time.h> to check if the "timeval" definition is
already in place. If it is, <exec/types.h> will not be included.

- <sys/time.h> can be made to define the timeval data structure locally if
the __USE_CLIB2_TIMEVAL preprocessor symbol is defined. In this case no
<exec/types.h> or <devices/timer.h> file will be read.

- The sprintf() family did not properly handle empty strings as format
specifications. This would result in a necessary buffer flush action
getting skipped, which consequently did not put the string termination
character into the output buffer.