sh_exec(): remove a vfork(2) use for the >; redirection

vfork has been removed from the POSIX standard and macOS gives a
deprecation warning for it.

Additionally, the old (2004) POSIX spec for it states:
https://pubs.opengroup.org/onlinepubs/009696799/functions/vfork.html
| The vfork() function shall be equivalent to fork(), except that
| the behavior is undefined if the process created by vfork()
| either modifies any data other than a variable of type pid_t used
| to store the return value from vfork(), or returns from the
| function in which vfork() was called, or calls any other function
| before successfully calling _exit() or one of the exec family of
| functions.

According to that spec, this vfork(2) usage was very much engaged
in undefined behaviour, as the first thing it does after vforking
is call _sh_fork(), and it never calls an exec function.

The lib_vfork feature test in src/lib/libast/features/lib does try
to verify that vfork works and does not merely exist, but it does
not attempt to run anything substantial in the child process
(besides, 'undefined behaviour' does not mean the test would
necessarily fail). That test may be good enough for the spawnveg(3)
vfork fallback, but it is inherently unreliable for determining if
we can run anything substantial in the child.

(This if(rewrite) code block is used when the >; redirection
operator (IOREWRITE) is used with an external command. It won't
work without the extra (v)fork, as the external command will be
replacing the ksh child process but ksh needs to decide if the
command succeeds before rewriting the output from a temp file.)

src/cmd/ksh93/include/shnodes.h

@@ -182,7 +182,7 @@ struct arithnod
 #define IOVNM		0x10000	/* iovname field is non-zero */
 #define IOLSEEK		0x20000	/* seek operators <# or >#  */
 #define IOARITH		0x40000	/* arithmetic seek <# ((expr))  */
-#define IOREWRITE	0x80000	/* arithmetic seek <# ((expr))  */
+#define IOREWRITE	0x80000	/* rewrite/truncate upon command success: >;word <>;word */
 #define IOCOPY		IOCLOB	/* copy skipped lines onto standard output */
 #define IOPROCSUB	IOARITH	/* process substitution redirection */
 

src/cmd/ksh93/sh/xec.c

@@ -44,12 +44,6 @@
 #   include	<vmalloc.h>
 #endif
 
-#if     _lib_vfork
-#   include     <ast_vfork.h>
-#else
-#   define vfork()      fork()
-#endif
-
 #if _lib_getrusage && !defined(RUSAGE_SELF)
 #   include <sys/resource.h>
 #endif
@@ -1633,7 +1627,7 @@ int sh_exec(const Shnode_t *t, int flags)
 				if(rewrite)
 				{
 					job_lock();
-					while((parent = vfork()) < 0)
+					while((parent = fork()) < 0)
 						_sh_fork(parent, 0, NULL);
 					if(parent)
 					{