Implement header preamble and reorder tags to match FreeBSD/NetBSD.

Modify mtree_set_option() so you can pass down a "0" or "1" to
disable/enable a specific option, or a string when the option allows
for a string value (which is the case for header).

Replaced the "#mtree" place-holder by the one generated by original
mtree, which specifies the user, machine, tree (path) and date
that generate the specification.

Implement options:
- "blanklines" (print blanklines between mtree entries or not -- matches
    -b flag for mtree(8));
- "dironly" (list only directories -- matches -d flag);
- "indent" (indent given tree depth -- matches -j flag);
- "comments" (print "# ..." comments -- matches -n flag).

Adapt the regression tests accordingly.

XXX the mtree-classic output does not match the ones generated by
FreeBSD/NetBSD mtree(8). Should be discussed with Tim first.
Implement the -b, -j, -d and -n flags. Modify the calls to
mtree_set_option.
Adapt regression tests to the expected, original FreeBSD/NetBSD output.