improved bbd.pod documentation

More examples, more links, some clarifications.
gpsman2bbd.pl: fixed a remaining call to BBBikeGPS::get_symbol_to_img

Replaced with a call to GPS::Symbols::Garmin.
show_mark: make blinking mark top-most

Reasoning: the blinking mark does not hide map elements permanently,
and vanishes after a few blinks anyway, so it can be raised above
everything. On the other hand, if the mark is steady, then use the
normal stack order (which is below most map layers, but above
area-like elements).
data update (u.a. additions in Eiche and Mehrow, and B2 -> L200)
bbd2mapservhtml.pl: clarify "XXX_prog: no_routelist" directive
bbd.pod: document new "#: ignore" directive

This will replace the current "#: section: ignorieren" directive.
HOWTO_edit_bbbike_data.*: replaced "section ignorieren" by "ignore" directive
some data files: "section ignorieren" -> "ignore"

Also changed "grepstrassen" calls in Makefiles.
fixed harmless typos