PROTON-660: Fix openssl.c build on windows

* openssl.c:
   - On windows we need to include winsock before openssl headers

* platform.h:
   - replace strcasecmp with !pn_i_eq_nocase
   - replace strncasecmp with !pn_i_eq_n_nocase
   - include winsock.h on windows
   - undefine symbols 'ERROR' and 'small' that proton codebase uses.

* util.c, util.h:
   - add pn_i_eq_n_nocase
   - remove static from pn_i_eq_nocase