merge: trivial code fixes

 * fix spelling
 * compare integers against 0
 * standardise naming in B64Context
 * standardise naming in ComposeRedrawData
 * standardise naming in FgetConv
 * standardise naming in Group
 * standardise naming in ImapHeader
 * standardise naming in MailboxNode
 * standardise naming in Pattern
 * standardise naming in SslSockData
 * standardise naming in Pager
 * simplify bitfields in Mailbox
 * simplify bitfields in ImapAccountData
 * simplify bitfields in smtp
 * drop bitfields
 * doxy: mark all out-params
 * tidy mutt_folder_hook
 * index.c move data
 * tidy reopen_mailbox
 * doxy: add comment blocks
 * iwyu
 * boolify variables
 * doxy: fix typo
 * fix param names
 * rename pop_adata_get
 * eliminate asymmetric quotes
 * reduce variable scope