clib2 Code

- Simplified the code in time_asctime_r.c which builds the time
string. It gracefully handles buffer sizes which are too short
by returning an empty string.

- Moved the tm->tm_wday initialization out of the hook function
in time_strftime.c since it was to be called only once anyway.

- Lost a few compiler warnings in unistd_time_delay.c and
time_gettimeofday.c.

- Folded duplicate code in time_mktime.c; also, errno is no longer
modified unless the library is built with the CHECK_FOR_NULL_POINTERS
option.