Revise the documentation of simple calendar time.

This is a thorough revision of all the material relating to the
functions time, stime, gettimeofday, settimeofday, clock_gettime,
clock_getres, clock_settime, and difftime, spilling over into the
discussion of time-related data types (which now get their own
section) and touching the adjtime family as well (which deserves its
own thorough revision, but I'd have to do a bunch of research first).

Substantive changes are:

 * Document clock_gettime, clock_getres, and clock_settime.  (Only
   CLOCK_REALTIME and CLOCK_MONOTONIC are documented; the others are
   either a bit too Linux-specific, or have more to do with measuring
   CPU/processor time.  That section _also_ deserves its own thorough
   revision but again I'd have to do a bunch of research first.)

 * Present gettimeofday, settimeofday, and struct timeval as obsolete
   relative to clock_*.

 * Remove the documentation of struct timezone.  Matching POSIX,
   say that the type of the second argument to gettimeofday and
   settimeofday is [const] void *.

 * Clarify ISO C and POSIX's requirements on time_t.  Clarify the
   circumstances under which difftime is equivalent to simple
   subtraction.

 * Consolidate documentation of most of the time-related data types
   into a new section "Time Types," right after "Time Basics."  (The
   exceptions are struct tm, which stays in "Broken-down Time," and
   struct times, which stays in "Processor And CPU Time."

 * The "Elapsed Time" section is now called "Calculating Elapsed Time"
   and includes only difftime and the discussion of how to compute
   timeval differences by hand.

 * Fold the "Simple Calendar Time," "High Resolution Calendar," and
   "High Accuracy Clock" sections together into two new sections titled
   "Getting the Time" and "Setting and Adjusting the Time."

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