merge: upstream fixes (mutt/default)

 * Fix POP3 SASL authentication mechanism DIGEST-MD5. (closes #3862)
 * Add a few explanatory comments to pop_auth_sasl().  (see #3862)
 * Fix GPGME signature zero timestamp and locale awareness issues. (closes #3882)
 * Handle presence of '--' delimiter in $sendmail. (closes #3168)
 * Allow IPv6 literal addresses in URLs. (closes #3681)
 * Fix gpgme segfault in create_recipient_set().
 * Use mutt_strlen and mutt_strncmp in sidebar.c.
 * Change sidebar to only match $folder prefix on a $sidebar_divider_char. (closes #3887)
 * Actually fix gpgme segfault in create_recipient_set().