merge: Upstream refactoring

 * Change struct Mailbox->realpath to be const char *
 * Convert struct Buffer->path to a struct Buffer
 * Convert other users of struct Mailbox->pathbuf to use Buffers
 * Add mutt_buffer_enter_fname()
 * Convert change folder operations to use struct Buffer
 * Increase prompt size to remove warning
 * Convert migrated lib.c functions to use struct Buffer
 * Add menu dialog helper to add rows
 * Convert certficate prompts to use menu dialog helper and struct Buffer
 * Convert pager help string to use struct Buffer
 * Convert compress to use mutt_buffer_quote_filename()
 * Convert remailer to use struct Buffer for Mixmaster invocation
 * Revert "Fix truncation of long filenames in attachments."
 * Handle the same secret key appearing in multiple public keyrings
 * Change mutt_parse_mailboxes() to use the path struct Buffer argument
 * Convert mutt_parse_adrlist() to use struct Buffer for simple parsing
 * Add note about $mailcap_path generation to the documentation
 * Add $imap_fetch_chunk_size to allow FETCHing new headers in chunks
 * Enable the use of toggle-write from the pager
 * Add note about IMAP browser and trailing delimiters
 * Add $forward_attachments quadoption for inline-forward mode
 * Note the other parts of generated message-ids in the manual
 * Fix mailbox search to not recompile for a repeated search
 * Corrected minor errors in text related to $forward_attachments
 * Update French translation
 * Write rfc2231 parameter continuations for long parameters
 * Add a manual section on OpenPGP and S/MIME configuration.
 * build: fix tests
 * tidy code