Re-enable SHOPT_DEVFD, fixing process substitution fd leaks (#218)

This commit fixes a long-standing bug (present since at least
ksh93r) that caused a file descriptor leak when passing a process
substitution to a function, or (if compiled with SHOPT_SPAWN) to a
nonexistent command.

The leaks only occurred when ksh was compiled with SHOPT_DEVFD; the
FIFO method was unaffected.

src/cmd/ksh93/sh/xec.c: sh_exec():
- When a process substitution is passed to a built-in, the
  remaining file descriptor is closed with sh_iorestore. Do the
  same thing when passing a process substitution to a function.
  This is done by delaying the sh_iorestore() call to 'setexit:'
  where both built-ins and functions terminate and set the exit
  status ($?).
  This means that call now will not be executed if a longjmp is
  done, e.g. due to an error in a special built-in. However, there
  is already another sh_iorestore() call in main.c, exfile(), line
  418, that handles that scenario.
- sh_ntfork() can fail, so rather than assume it will succeed,
  handle a failure by closing extra file descriptors with
  sh_iorestore(). This fixes the leak on command not found with
  SHOPT_SPAWN.

src/cmd/ksh93/include/defs.h:
- Since the file descriptor leaks are now fixed, remove the
  workaround that forced ksh to use the FIFO method.

src/cmd/ksh93/SHOPT.sh:
- Add SHOPT_DEVFD as a configurable option (default: probe).

src/cmd/ksh93/tests/io.sh:
- Add a regression test for the 'not found' file descriptor leak.
- Add a test to ensure it keeps working with 'command'.

Fixes: https://github.com/ksh93/ksh/issues/67

src/cmd/ksh93/include/defs.h

@@ -43,14 +43,6 @@
 #   define mbmax()	1
 #endif
 
-/*
- * Until a file descriptor leak with process substitution
- * is fixed, disable /dev/fd use to avoid the problem.
- * https://github.com/ksh93/ksh/issues/67
- */
-#undef SHOPT_DEVFD
-#define SHOPT_DEVFD	0
-
 #include	<sfio.h>
 #include	<error.h>
 #include	"FEATURE/externs"