Move libapps headers into their own directory

This got triggered by test/testutil.h including ../apps/opt.h.

Some compilers do all inclusions from the directory of the C file
being compiled, so when a C file includes a header file with a
relative file spec, and that header file also includes another header
file with a relative file spec, the compiler no longer follows.

As a specific example, test/testutil/basic_output.c included
../testutil.h.  Fine so far, but then, test/testutil.h includes
../apps/opt.h, and the compiler ends up trying to include (seen from
the source top) test/apps/opt.h rather than apps/opt.h, and fails.

The solution could have been to simply add apps/ as an inclusion
directory.  However, that directory also has header files that have
nothing to do with libapps, so we take this a bit further, create
apps/include and move libapps specific headers there, and then add
apps/include as inclusion directory in the build.info files where
needed.

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
(Merged from https://github.com/openssl/openssl/pull/8210)