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