merge: upstream fixes (mutt/default)

 * search muttrc file according to XDG Base Specification (closes #3207)
 * Fix loop terminator in in mutt_find_cfg().
 * Backout inlining of MUTT_VERSION in 42fee7585fae.
 * Revert changes made to mutt_find_cfg() in 3c6d322912e3
 * Improve openssl interactive_check_cert. (closes #3899)
 * Add mutt_array_size macro, change interactive_check_cert() to use it. (see #3899)
 * Return to pager upon aborting a jump operation. (closes #3901)
 * Change sidebar_spoolfile coloring to be lower precedence.
 * automatic post-release commit for mutt-1.7.2
 * Move '@' pattern modifier documentation to the right section.
 * Add setenv/unsetenv commands.
 * Rework OpenSSL certificate verification to support alternative chains. (closes #3903)
 * Remove unused OPS
 * Add option to control whether threads uncollapse when new mail arrives.
 * In the manual, replaced 2 para by example (similar to the first example).
 * Create mbchar_table type for multibyte character arrays. (see #3024)
 * Make to_chars and status_chars accept mulitibyte characters. (closes #3024)
 * sync xdg config file handling
 * sync Fromchars to upstream version