replace 'unsigned const char' with 'const unsigned char'

replace 'unsigned const char' with 'const unsigned char'

The openssl code base has only a few occurrences of 'unsigned const char'
(15 occurrences), compared to the more common 'const unsigned char' (4420
occurrences).

While the former is not illegal C, mixing the 'const' keyword (a 'type
qualifier') in between 'unsigned' and 'char' (both 'type specifiers') is a
bit odd.

The background for writing this patch is not to be pedantic, but because
the 'opmock' program (used to mock headers for unit tests) does not accept
the 'unsigned const char' construct. While this definitely is a bug in
opmock or one of its dependencies, openssl is the only piece of software we
are using in combination with opmock that has this construct.

CLA: trivial

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/13722)