flock: fix hang when parent ignores SIGCHLD

flock: fix hang when parent ignores SIGCHLD

If flock is executed from a process which has set SIGCHLD to SIG_IGN, then
flock will eat cpu and hang indefinitely if given a command to execute.
So before we fork(), make sure to set SIGCHLD handling back to the default
so that the later waitpid() doesn't freak out on us.

[kzak@redhat.com: - add a check for waitpid() return value]

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Karel Zak <kzak@redhat.com>