posix_spawn: use larger stack to cover worst-case in execvpe

execvpe stack-allocates a buffer used to hold the full path
(combination of a PATH entry and the program name)
while searching through $PATH, so at least
NAME_MAX+PATH_MAX is needed.

The stack size can be made conditionally smaller
(the current 1024 appears appropriate)
should this larger size be burdensome in those situations.