posix_spawn_file_actions_addopen needs to copy the path argument (BZ 17048)

POSIX requires that we make a copy, so we allocate a new string
and free it in posix_spawn_file_actions_destroy.

Reported by David Reid, Alex Gaynor, and Glyph Lefkowitz.  This bug
may have security implications.

(cherry picked from commit 89e435f3)

https://bugs.gentoo.org/513090
https://sourceware.org/bugzilla/show_bug.cgi?id=17048