random: Make sure to re-open /dev/random after a fork

* random/rndlinux.c (_gcry_rndlinux_gather_random): Detect fork and
re-open devices.

This mitigates about ill-behaving software which has closed the
standard fds but later dups them to /dev/null.

(cherry picked from commit 319f55e6e5793c59f1ba4cfe481b562bca42194d)

• GnuPG-bug-id: T3491
• Signed-off-by: Werner Koch <wk@gnupg.org>