camel-sasl-gssapi: Generate correct error messages for non-krb5 mechanisms

This code is littered with assumptions that GSSAPI means krb5. Let's fix
at least some of them, since HTTP Negotiate means SPNEGO, not just krb5.

Discussed as part of bug 732850.

[dwmw2: Some fixes and refactoring of Daniel's original patch]
camel-sasl-gssapi: Use SPNEGO for HTTP auth not bare krb5

Discussed as part of bug 732850
Improve error on failed conversion

Not a lot of point in showing the error that we got on converting the
original error. Just show one number.

Not that this should ever happen, hopefully.
camel-sasl-gssapi: Always use gss_display_status() for mechanism error code
Bug 732627 - [IMAPx] Add support for literal8 in nstring
Share subprocesses for built-in backends by default

It turned out that opening subprocesses for each single instance
is a very CPU and resource expensive operation, which can be seen
for example when opening a meeting invitation email, which does
a flood of open requests to open each configured calendar (and
a Birthdays & Anniversaries calendar opens some books). Using one
subprocess for a backend type makes it use less resources.
NEWS update for 3.13.4
Post-release version bump
[IMAPx] Add actual error message into 'Select failed' debug log
update zh_CN translation
update zh_CN translation
doap category core
doap: add <programming-language>
Updated Basque language