Stop the time keyword overriding errexit (#351)

This bug was first reported in <https://www.illumos.org/issues/7694>.
The time keyword currently overrides the errexit shell option,
allowing failing scripts to continue after an error:

  $ cat 1.sh
  #!/bin/sh
  time false   # This should cause the script to exit
  echo FAILURE
  true
  $ ksh -o errexit 1.sh

  real    0m0.00s
  user    0m0.00s
  sys     0m0.00s
  FAILURE

src/cmd/ksh93/sh/xec.c:
- When the time keyword runs a command, pass the errexit state flag
  to the sh_exec call. This state flag is required for ksh to exit
  when a command fails while the errexit option is on.

src/cmd/ksh93/tests/basic.sh:
- Add a regression test based on the reproducer.

src/cmd/ksh93/sh/xec.c

@@ -2404,7 +2404,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 #endif
 				job.waitall = 1;
 				sh_onstate(SH_TIMING);
-				sh_exec(t->par.partre,OPTIMIZE);
+				sh_exec(t->par.partre,sh_isstate(SH_ERREXIT)|OPTIMIZE);
 				if(!timer_on)
 					sh_offstate(SH_TIMING);
 				job.waitall = 0;