Revert "Move random-related defines from e_os.h to rand_unix.c"

Revert "Move random-related defines from e_os.h to rand_unix.c"

This reverts commit 7b18d1a53f932391bbc599a4717d6f98a597849c, which moved the
DEVRANDOM and DEVRANDOM_EGD defines into rand_unix.c. That change introduced
the regression that the compiler complains about missing declarations in
apps/version.c when OpenSSL is configured using `--with-rand-seed=devrandom`
(resp. `--with-rand-seed=egd`):

apps/version.c:173:42: error: 'DEVRANDOM' undeclared
             static const char *dev[] = { DEVRANDOM, NULL };
                                          ^~~~~~~~~

Fixes #10759

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10764)