- enable compiler warning `-Wcast-qual`.
- add `LIBSSH2_UNCONST()` macro to strip const where absolutely
necessary to avoid compiler warnings.
- fix const stripping by constifying where necessary.
- fix const stripping by using `LIBSSH2_UNCONST()`.
- libgcrypt.h: drop unnecessary casts.
- openssl: fix to use new `BIO_new_mem_buf()` parameter types
with wolfSSL.
Cherry-picked from #1484Closes#1527