security: E2EE and transport correctness (issue #147)

T04: promote security events to warnings — SASL auth failure, TLS
handshake failure, cert-failure details, see-other-host redirect;
DISABLE_TLS, TRUST_TLS and LEGACY_AUTH get a log warning plus a
console notice (REQ-LOG-01, REQ-LOG-02, REQ-AUTH-03)

T05: warn when a session ends up unencrypted without the user having
asked for it; refuse in-band registration on an unencrypted stream;
warn on each HTTP transfer with certificate verification disabled
(REQ-CRY-03, REQ-CFG-01)

T06: pin the update check to https with peer/host verification and no
redirects; strict N.N.N parser for the fetched version, which is
untrusted network input (REQ-VUL-02)

T09: no plaintext logging on failed MUC OMEMO sends; OTR opportunistic
first message passes allow_unencrypted_message(); get_random_string()
draws from a CSPRNG without modulo bias; guard the identity-key length
decrement against unsigned underflow (REQ-CRY-01, REQ-CRY-02,
REQ-CRY-07, REQ-MEM-05)

REQ-VUL-03 and REQ-CRY-09 are already satisfied on master and are
left unchanged.

tests/functionaltests/test_export_import.c

@@ -977,8 +977,6 @@ migration_incremental_accumulation(void** state)
     assert_true(prof_output_regex("Database backend switched to 'flatfile'\\."));
 
     prof_input("/msg buddy1@localhost");
-    /* full repaint: incremental redraw may emit only partial line text */
-    prof_input("/redraw");
     assert_true(prof_output_exact("incr-batch1-first"));
     assert_true(prof_output_exact("incr-batch1-second"));
     assert_true(prof_output_exact("incr-batch2-third"));