merge: upstream fixes (mutt/default)

 * Canonicalize line endings for GPGME S/MIME encryption. (closes #3904)
 * Add a pattern_cache_t to speed up a few repeated matches.
 * Fix imap server-side search to call uid2msgno() only once. (see #3905)
 * Convert HASH to be indexable by unsigned int. (see #3905)
 * Create a uid hash for imap. (see #3905)
 * Convert cmd_parse_search to use the uid hash. (closes #3905)
 * Fix minor documentation issues.
 * Allow initial blank lines in local mailboxes.
 * Improve error handling in mbox magic detection.
 * Add casecmp and strdup_key flags to hash_create()
 * Fix several alias hashtable issues.
 * Fix sidebar references to the "new count" to be "unread". (closes #3908)
 * Improve Reply-to vs From comparison when replying. (closes #3909)
 * Abstract the SPAM_LIST as a generic REPLACE_LIST
 * Add subjectrx command to replace matching subjects with something else.
 * Allow "unsubjectrc *" to remove all patterns.
 * Adds capability to edit x-labels inside mutt, and to sort by label.
 * Minor fixes to the x-label patch from David.
 * Add hash_find_elem to get the hash element.
 * Adds label completion.
 * Improve the label completion hash table usage.
 * Fix the x-label update code check location.
 * Fix the mutt_label_complete() pos parameter.
 * Permit tab completion of pattern expressions with ~y (labels).
 * Simplify mutt_label_complete().