fix display of message on TOFU_FINGERPRINT_MISMATCH

Previously the message was never displayed to users
leaving them with a simple "Error: certificate is untrusted".

This also fixes the display of line numbers in the message.

Fix OpenBSD compilation errors

Those changes fix the following compilation errors on OpenBSD:

src/tofu.c:128:28: error: format specifies type 'long' but the argument has type
      'time_t' (aka 'long long') [-Werror,-Wformat]
                        "SHA-512", fingerprint, expires);

src/gmnlm.c:341:31: error: missing sentinel in function call
      [-Werror,-Wsentinel]
                execlp("sh", "sh", "-c", cmd);
                                            ^
                                            , NULL

Fix more strncpy bugs in gmnlm and tofu

>From gcc 9.3.0:

error: '__builtin_strncpy' specified bound 4097 equals destination size
[-Werror=stringop-truncation]

Signed-off-by: William Casarin <jb55@jb55.com>

Fix bug on mkdirs calls

On some systems dirname uses a static string for its return value, so
we were calling mkdirs recursively on a string that was continuosly changing.

A check was also added after the `snprintf` to make sure there's no
information loss since there is no limit to the length of the string
returned by `get_data_pathfmt`.

Closes #48.

Add static library for gmni

Add 'd' to download page

Init known_hosts prior to fopen to avoid segfault.

Fix truncated hashes in known_hosts

tofu.c: clarify reuse warning

TOFU: verify hostnames

TOFU: use ~/.local/share/gemini/known_hosts

The rest of gmni's stuff will remain in ~/.local/share/gmni, but in
order to establish a common location and format for the TOFU file
between Gemini implementations, a more general path is required.

Fix memory leaks

Fix issues with tofu.c

Implement TOFU