[Toybox] About 2/3 of mount checked in.

[Rob Landley]

It's got basic flag parsing and is autodetecting bind and loopback
mounts, but no support for remount or fstab yet.

You may notice the new xpopen() infrastructure in lib, which shells out
to an xexec() with stdin/stdout/stderr captured via pipes. That's a
thing I've needed for a while so we can fork and call other local
commands without needing an exec. (Doesn't do xexec_optargs() but I
haven't thought of a case needing i/o capture for that yet.)

Not quite sure that's what the final infrastructure should look like,
since $(blah) in shells also needs something like that, but in that case
it _shouldn't_ capture stderr and I'm not sure what it should do with
stdin either...

  $ echo hello | echo $(cat)
  hello

Looks like $() should leave stdin alone? I should come up with some way
to specify...

Oh, and there's some other general infrastructure work recently:
generic_signal() handler, llist_free functions for the predefined types,
dlist_terminate(), and xfork(). Not sure I've converted everything that
can take advantage of those yet. (I know I haven't got all the
generic_signal() stuff, need to frown at the pipe notification mechanism
still...)

I need to update the documentation. I'm getting to the point where
there's enough material I might need separate tutorial and reference
versions, except keeping duplicate copies of the same information in
sync is never fun. Maybe I can leverage toys/example harder...

Rob

(It's singular/plural names that really screw me up. lib is singular,
scripts is plural, toys is plural, toys/example is singular... Sigh. I
can never remember if various websites have "download" or "downloads",
but then my own website has "writing" singular "and "talks" plural...)