Use fork()/exec() instead of posix_spawn() by default

Using posix_spawn() may result in a race condition while setting
terminal foreground process group. Copying dgk's response from related
discussion in ast-developers mailing list:

"
ksh93 uses posix_spawn() rather than fork/exec to run a simple command
for performance reasons.  However, posix_spawnv() doesn't have a way to
set the terminal group before the exec().  The parent process sets the
terminal group which leads to the race.

There is a simple change you can make to a program to guarentee that
the terminal will get set at the beginning of the program.
You can add two lines.  One line is
        #include        <termios.h>

and for the other line
        tcsetpgrp(2,getpgrp());

Alternatively, you can write a program that does these two lines and then
execs the original program.
"

Only way to reliably fix this problem in ksh is to switch back to fork()/
exec().

If there are any performance regressions or bugs caused by this change,
you can switch to posix_spawn() by setting '-D_AST_no_spawnveg=0'.

Related: #468
Related: rhbz#1160482

meson.build

@@ -66,6 +66,7 @@ endif
 
 shared_c_args = [
     '-DUSAGE_LICENSE=""',
+    '-D_AST_no_spawnveg=1',
 ]
 
 # Compiler arguments used when building `ksh93`, `shcomp`, and related AST