merge: bug fixes

 * Selectively cache headers
 * Don't overflow stack buffer in msg_parse_fetch
 * Handle NO response without message properly
 * Ensure litlen isn't larger than our mailbox
 * Check for int underflow in imap_quote_string
 * imap_quote_string make room for quotes
 * Ensure UID in fetch_uidl
 * Truncate pct-encoded strings
 * Check outbuf length in mutt_to_base64()