Instead, generate new keys, and overwrite the empty key files.
Adds FN_EMPTY to file_status_t and file_status.
Fixes bug 13111.

Related changes due to review of FN_FILE usage:
Stop generating a fresh .old RSA key file when the .old file is missing.
Avoid overwriting .old key files with empty key files.
Skip loading zero-length extra info store, router store, stats, state,
and key files.

Stop crashing when a NULL filename is passed to file_status(),
instead, return FN_ERROR.
Also return FN_ERROR when a zero-length filename is passed to file_status().
Fixed as part of bug 13111.

Needed for #13399.

Silence clang warnings under --enable-expensive-hardening, including:
  + implicit truncation of 64 bit values to 32 bit;
  + const char assignment to self;
  + tautological compare; and
  + additional parentheses around equality tests. (gcc uses these to
    silence assignment, so clang warns when they're present in an
    equality test. But we need to use extra parentheses in macros to
    isolate them from other code).

Also, tweak it so that it puts major deprecations and requirements
early in the changelog.

(Also resort and rewrap)

Because in 95 years, we or our successors will surely care about
enforcing the BSD license terms on this code.  Right?

I concatenated the remaining changes/* files, removed them, made the
headings more uniform, then told format_changelog.py to sort,
collate, and wrap them.

(This means that changes/* is now "everything that changed since
0.2.5.10".)

By now, support in the network is widespread and it's time to require
more modern crypto on all Tor instances, whether they're clients or
servers. By doing this early in 0.2.6, we can be sure that at some point
all clients will have reasonable support.

When Tor first generates identity keypair, emit a log message that
thanks for their participation and points to new Tor relay lifecycle
document.

Arma found this and commented on #11243.  Bug not in any released
version of Tor.

Otherwise, we get implicit conversion warning on some platforms.