... and a few more cleanups/clarifications

Global symbol "$mailbox" requires explicit package name

...as this will be required for IMAP CLOSE and EXPUNGE commands as well
as the POP3 RSET command.

Solaris with the SunStudio Compiler is reportedly missing this define,
but as we're using it without any good reason on all the places it was
used I've now instead switched to just use sensible buffer sizes that
fit a 32 bit decimal number. Which also happens to be smaller than the
common NI_MAXSERV value which is 32 on most machines.

Bug: http://curl.haxx.se/bug/view.cgi?id=1277
Reported-by: D.Flinkmann

... instead of HTTP/2.0 to work fine with the nghttpx proxy/server.

warning: comparison between signed and unsigned integer expressions

Don't wait for the next callback call (usually 1 second) before
continuing with protocol specific connection initialization.

This function was modified in nghttp2 git commit a1c3f89c72e51

Otherwise, the FTP protocol would unnecessarily hang 60 seconds if
aborted in the CURLOPT_HEADERFUNCTION callback.

Reported by: Tomas Mlcoch
Bug: https://bugzilla.redhat.com/1005686

Make sure that the custom struct fields are only used by code that
doesn't use a struct defintion from the outside.

Attempts to fix the problem introduced in 3dc6fc42

Otherwise the connection can get stuck during various phases, waiting
for new data on the socket using select() etc., but it will never be
received as the data has already been read into SSL library.

The transfer size would be calculated incorrectly if the email contained
within the FETCH response, had been partially received by the pingpong
layer. As such the following, example output, would be seen if the
amount remaining was smaller than the amount received:

* Excess found in a non pipelined read: excess = 1394, size = 262,
  maxdownload = 262, bytecount = 1374
* transfer closed with -1112 bytes remaining to read

Bug: http://curl.haxx.se/mail/lib-2013-08/0170.html
Reported-by: John Dunn

...from commits 28427b40 and e8313697.

Renamed SUPPORTAUTH to AUTH and added support for specifying a list of
supported SASL mechanisms to return to the client.

Additionally added the directive to the FILEFORMAT document.

Renamed SUPPORTCAPA to CAPA and added support for specifying a list of
supported capabilities to return to the client.

Additionally added the directive to the FILEFORMAT document.

The message numbers given in the LIST response are an index into the
list, which are only valid for the current session, rather than being a
unique message identifier. An index would only be missing from the LIST
response if a DELE command had been issued within the same session and
had not been committed by the end of session QUIT command. Once
committed the POP3 server will regenerate the message numbers in the
next session to be contiguous again. As such our LIST response should
list message numbers contiguously until we support a DELE command in the
same session.

Should a POP3 user require the unique message ID for any or all
messages then they should use the extended UIDL command. This command
will be supported by the test ftpserver in an upcoming commit.