This commit backports the required fixes from ksh2020 for using
millisecond precision with the 'time' keyword. The bugfix
refactors a decent amount of code to rely on the BSD 'timeradd'
and 'timersub' macros for calculating the total amount of time
elapsed (as these aren't standard, they are selectively
implemented in an iffe feature test for platforms without them).
getrusage(3) is now preferred since it usually has higher precision
than times(3) (the latter is used as a fallback).

This patch can be extended with support for microsecond precision,
although that is better left for a separate feature branch.
There are three other fixes as well:

src/lib/libast/features/time:
- Test for getrusage with an iffe feature test rather than
  assume _sys_times == _lib_getrusage.

src/cmd/ksh93/sh/xec.c:
- A single percent at the end of a format specifier is now
  treated as a literal '%' (like in Bash).
- Zero-pad seconds if seconds < 10. This was already done for
  the times builtin in commit 5c677a4, although it wasn't
  applied to the time keyword.
- Backport the ksh2020 bugfix for the time keyword by using
  timeradd and timersub with gettimeofday (which is used with
  a timeofday macro). Prefer getrusage when it is available.

src/cmd/ksh93/features/time:
- Implement feature tests for the 'timeradd' and 'timersub'
  macros.
- Do a feature test for getrusage like in the libast time test.

src/cmd/ksh93/tests/basic.sh:
- Add two tests for millisecond precision and the handling of
  '%' at the end of a format specifier.

src/cmd/ksh93/sh/xec.c:
- Allow compiling without the 'timeofday' ifdef for better portability.
  This is the order of priority for getting the elapsed time:
  1) getrusage (most precise)
  2) times + gettimeofday (best fallback)
  3) only times (doesn't support millisecond precision)
  This was tested by using debug '#undef' statements in xec.c.

src/cmd/ksh93/sh/xec.c:
- Restore some old code for the fallback when _lib_getrusage
  and timeofday aren't defined to fix a bus error.

src/cmd/ksh93/sh/xec.c:
- In subshells, getrusage must be run after forking. If it isn't,
  'time' can return negative user and system times.

src/cmd/ksh93/tests/basic.sh:
- Modify the radix regression test to check for negative numbers
  as well.