Merge branch 'nss_squashed' into nss_merge

changes/NSS

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+  o Major features (portability, cryptography, experimental, TLS):
+    - Tor now has the option to compile with the NSS library instead of
+      OpenSSL. This feature is experimental, and we expect that bugs may
+      remain. It is mainly intended for environments where Tor's performance
+      is not CPU-bound, and where NSS is already known to be installed.
+      To try it out, configure Tor with the --enable-nss flag.
+      Closes ticket 26631.

changes/feature26815

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+  o Major features (experimental, library support):
+    - When built with --enable-nss, Tor now uses the NSS library for digests,
+      AES, and pseudorandom numbers. Closes ticket 26815.

changes/feature26816

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+  o Major features (experimental, library support):
+    - Tor now has _partial_ support for using the NSS cryptography library in
+      place of OpenSSL.  When Tor is configured with --enable-nss, it will
+      use NSS for several (but not yet all) of its cryptography.  (It still
+      relies on OpenSSL for the rest.)  Eventually, if all goes as planned,
+      "--enable-nss" will produce a version of Tor that does not depend on
+      OpenSSL.  Implements ticket 26816.
+
+      WARNING: This feature is experimental.  Don't use it for real security
+      yet, until the code has had much more review, and more bugs have been
+      shaken out.