`try_clone` does not set CLOEXEC

[purplesyringa]

try_clone calls dup(2), which does not set the CLOEXEC flag. fcntl(2) with command F_DUPFD_CLOEXEC should be used instead.

[cehteh]

Yay, another can of worms. A short search reveals that F_DUPFD_CLOEXEC is not supported on all unixes/clibs (despite being POSIX). I will add a feature/check for that on my fork next days.