imap: Fixed incorrect fallback to clear text authentication

If a specific SASL authentication mechanism was requested by the user
as part of the login options but wasn't supported by the server then
curl would fallback to clear text, when it shouldn't, rather than
reporting "No known authentication mechanisms supported" as the POP3
and SMTP protocols do.
imap: Fixed exclude of clear text when using auth=* in commit 75cd7fd66762bb

It is not 100% clear whether * should include clear text LOGIN or not
from RFC-5092, however, including it is then consistent with current
POP3 behaviour where clear text, APOP or SASL may be chosen.
login options: remove the ;[options] support from CURLOPT_USERPWD

To avoid the regression when users pass in passwords containing semi-
colons, we now drop the ability to set the login options with the same
options. Support for login options in CURLOPT_USERPWD was added in
7.31.0.

Test case 83 was modified to verify that colons and semi-colons can be
used as part of the password when using -u (CURLOPT_USERPWD).

Bug: http://curl.haxx.se/bug/view.cgi?id=1311
Reported-by: Petr Bahula
Assisted-by: Steve Holme
Signed-off-by: Daniel Stenberg <daniel@haxx.se>
curl_easy_setopt: clarify some USERPWD and PROXYUSERPWD details
RELEASE-NOTES: synced with dd4d9ea542
multi: add timer inaccuracy margin to timeout/connecttimeout

Since all systems have inaccuracy in the timeout handling it is
imperative that we add an inaccuracy margin to the general timeout and
connecttimeout handling with the multi interface. This way, when the
timeout fires we should be fairly sure that it has passed the timeout
value and will be suitably detected.

For cases where the timeout fire before the actual timeout, we would
otherwise consume the timeout action and still not run the timeout code
since the condition wasn't met.

Reported-by: He Qin
Bug: http://curl.haxx.se/bug/view.cgi?id=1298
curl.1 document -J doesn't %-decode

...also added as KNOWN_BUG #87 with reference to bug #1294