Merge two bugfixes in ASN1_STRING_TABLE_add(3) and ASN1_STRING_TABLE_get(3)

Merge two bugfixes in ASN1_STRING_TABLE_add(3) and ASN1_STRING_TABLE_get(3)
from the OpenSSL 1.1.1 branch, which is still under a free license,
mostly this commit:

commit d35c0ff30b31be9fd5dcf3d552a16feb8de464bc
Author: Dr. Stephen Henson <steve@openssl.org>
Date: Fri Oct 19 15:06:31 2012 +0000
fix ASN1_STRING_TABLE_add so it can override existing string table values

This fixes a segfault in ASN1_STRING_TABLE_add(3), which tried to change a
static const entry when called with an nid already in the default table,
and it switches the precedence of the two tables in ASN1_STRING_TABLE_get(3).

In addition, it changes behaviour in the following minor ways:
* Ignore negative minsize and maxsize arguments, not just -1.
* Ignore a zero mask and zero flags.
It's unclear whether these additional changes make the API absolutely
better, but we want compatibility with OpenSSL in these functions.

Tweaks & OK tb@.