fstrim: sync with busybox master (now an official applet)

fstrim: Needs to fire at the mp, not bd

It's a filesystem thing from this perspective.
+1b

fstrim: Indicate failure on error
+14b

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
fstrim: use new-style config/kbuild/applet snippets; trim help text

function                                             old     new   delta
packed_usage                                       29546   29520     -26

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>