Check return values of time functions

Check return values of time functions

Where ctime(), localtime(), or localtime_r() is used, check for
failure even if it is unlikely (reported by Bean Zhang).  Constify the
strdate() return type in kdb5_mkey.c and kadmin.c and the
ctime_uint32() return type in kproplog.c.  Use localtime_r()
unconditionally in str_conv.c as there is already a wrapper in that
file for the case where the platform doesn't have it.  Remove an
inoperative localtime() call in ktutil.c.