Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4217)

Apart from ssltest_old.c, the test suite relied on e_os.h for the
OSSL_NELEM macro and nothing else.

The ssltest_old.c also requires EXIT and some socket macros.

Create a new header to define the OSSL_NELEM macro and use that instead.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4186)

to halves MSVC warnings.
Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4172)

Documentation and test cases are also updated
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3783)