shell_hook: don't run command in "sh -c"
shell_hook: don't run command in "sh -c"

Using "sh -c" to run a command spawns a useless shell process and could
potentially be dangerous if the escaping was not properly done (which
should not be the case here though). Moreover, it is still possible to
spawn a shell by adding "sh -c '/a/command'" as template.

tftp-go-team/hooktftp,jumanjiman/hooktftp,moul/hooktftp