MFC 348206,348231,348454: GELI crypto deprecation warnings.

348206:
Add deprecation warnings for weaker algorithms to geli(4).

• Triple DES has been formally deprecated in Kerberos (RFC 8429) and is soon to be deprecated in IPsec (RFC 8221).
• Blowfish is deprecated. FreeBSD doesn't support its successor (Twofish).
• MD5 is generally considered a weak digest that has known attacks.

geli refuses to create new volumes using these algorithms via 'geli
init'. It also warns when attaching to existing volumes or creating
temporary volumes via 'geli onetime' . The plan is to fully remove
support for these algorithms in FreeBSD 13.

Note that none of these algorithms have ever been the default
algorithm used by geli(8). Users would have had to explicitly select
these algorithms when creating volumes in the past.

348231:
Correct the argument passed to g_eli_algo2str()

348454:
Remove tests for the deprecated algorithms in r348206

The tests are failing because the return value and output have changed, but
before test code structure adjusted, removing these test cases help people
be able to focus on more important cases.

Approved by: re (gjb)
Relnotes: yes