metalink: fix NSS issue in MultiSSL builds

In MultiSSL mode (i.e. when more than one SSL backend is compiled
in), we cannot use the compile time flag `USE_NSS` as indicator that
the NSS backend is in use. As far as Metalink is concerned, the SSL
backend is only used for MD5, SHA-1 and SHA-256 calculations,
therefore one of the available SSL backends is selected at compile
time, in a strict order of preference.

Let's introduce a new `HAVE_NSS_CONTEXT` constant that can be used
to determine whether the SSL backend used for Metalink is the NSS
backend, and use that to guard the code that wants to de-initialize
the NSS-specific data structure.

Ref: https://github.com/curl/curl/pull/1848

src/tool_metalink.c

@@ -30,6 +30,8 @@
 #  include <fcntl.h>
 #endif
 
+#undef HAVE_NSS_CONTEXT
+
 #ifdef USE_OPENSSL
 #  include <openssl/md5.h>
 #  include <openssl/sha.h>
@@ -50,6 +52,7 @@
 #  define MD5_CTX    void *
 #  define SHA_CTX    void *
 #  define SHA256_CTX void *
+#  define HAVE_NSS_CONTEXT
    static NSSInitContext *nss_context;
 #elif defined(USE_POLARSSL)
 #  include <polarssl/md5.h>
@@ -967,7 +970,7 @@ void clean_metalink(struct OperationConfig *config)
 
 void metalink_cleanup(void)
 {
-#ifdef USE_NSS
+#ifdef HAVE_NSS_CONTEXT
   if(nss_context) {
     NSS_ShutdownContext(nss_context);
     nss_context = NULL;