posix: execvpe cleanup

posix: execvpe cleanup

This patch removes all the dynamic allocation on execvpe code and
instead use direct stack allocation.  This is QoI approach to make
it possible use in scenarios where memory is shared with parent
(vfork or clone with CLONE_VM).

For default process spawn (script file without a shebang), stack
allocation is bounded by NAME_MAX plus PATH_MAX plus 1.  Large
file arguments returns an error (ENAMETOOLONG).  This differs than
current GLIBC pratice in general, but it used to limit stack
allocation for large inputs.  Also, path in PATH environment variable
larger than PATH_MAX are ignored.

The shell direct execution exeception, where execve returns ENOEXEC,
might requires a large stack allocation due large input argument list.

Tested on i686, x86_64, powerpc64le, and aarch64.

	* posix/execvpe.c (__execvpe): Remove dynamic allocation.
	* posix/Makefile (tests): Add tst-execvpe{1,2,3,4,5,6}.
	* posix/tst-execvp1.c (do_test): Use a macro to call execvp.
	* posix/tst-execvp2.c (do_test): Likewise.
	* posix/tst-execvp3.c (do_test): Likewise.
	* posix/tst-execvp4.c (do_test): Likewise.
	* posix/tst-execvpe1.c: New file.
	* posix/tst-execvpe2.c: Likewise.
	* posix/tst-execvpe3.c: Likewise.
	* posix/tst-execvpe4.c: Likewise.
	* posix/tst-execvpe5.c: Likewise.
	* posix/tst-execvpe6.c: Likewise.