[3.15] gh-40038: Quote imaplib command arguments when necessary (GH-152703) (GH-153066)

Argument quoting was inadvertently disabled when imaplib was ported to
Python 3 (bpo-1210 commented out the ``_checkquote()`` call, bpo-9638
then removed it), so since Python 3.0 commands failed for arguments
containing protocol-sensitive characters, such as a space in a mailbox
name.

Quoting is restored and reimplemented per the RFC 3501 grammar, so that
arguments that need quoting are escaped and quoted, while flags, sequence
sets and list wildcards are left intact.

For backward compatibility, an argument already enclosed in double quotes
is left unchanged, so code that quotes arguments itself keeps working.
(cherry picked from commit 600e864)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/imaplib.rst

@@ -186,6 +186,9 @@ enclosed with either parentheses or double quotes) each string is quoted.
 However, the *password* argument to the ``LOGIN`` command is always quoted. If
 you want to avoid having an argument string quoted (eg: the *flags* argument to
 ``STORE``) then enclose the string in parentheses (eg: ``r'(\Deleted)'``).
+In general, pass arguments unquoted and let the module quote them as needed.
+An argument that is already enclosed in double quotes is left unchanged,
+so that code which quotes arguments itself keeps working.
 
 Most commands return a tuple: ``(type, [data, ...])`` where *type* is usually
 ``'OK'`` or ``'NO'``, and *data* is either the text from the command response,
@@ -394,7 +397,7 @@ An :class:`IMAP4` instance has the following methods:
    .. versionadded:: 3.14
 
 
-.. method:: IMAP4.list([directory[, pattern]])
+.. method:: IMAP4.list(directory='', pattern='*')
 
    List mailbox names in *directory* matching *pattern*.  *directory* defaults to
    the top-level mail folder, and *pattern* defaults to match anything.  Returned
@@ -424,7 +427,7 @@ An :class:`IMAP4` instance has the following methods:
       The method no longer ignores silently arbitrary exceptions.
 
 
-.. method:: IMAP4.lsub(directory='""', pattern='*')
+.. method:: IMAP4.lsub(directory='', pattern='*')
 
    List subscribed mailbox names in directory matching pattern. *directory*
    defaults to the top level directory and *pattern* defaults to match any mailbox.