blkdiscard: avoid asserting when passed a bsd disklabel raw device

PR#57856 shows when using blkdiscard on eg, /dev/ld0 it asserts because
'0' is not between 'a' and 'p'. switch this to using DISKPART() on the
returned st_rdev, so it works on 'ld0c' or 'ld0' (rawpart=2.)

branches: 1.2.2;
fix various typos in documentation and comments.
mainly in words functionality, functional, function.

convert my fdiscard-stuff.c into blkdiscard.c and give it a UI
that matches the linux-util version.

comes in two forms: blkdiscard equivalent, and the (almost)
original interface i used (the "-s" option is conflicting and
i've switched to only the not yet implemented secure erase
option.) the only real difference is that "fdiscard" mode
requires the -R flag to do something, vs blkdiscard needs the
-n flag to not do anything.


not yet hooked into the build or sets. would like more testing
on it to be performed before doing that (but soon.)