[lmi] Easier cygwin update?

[Greg Chicares]

After studying something Stéphane Chazelas wrote, I thought
I could write a POSIX script better if only I had 'bc'. But
that tool hadn't been a prerequisite for lmi, so I added it
here:
  git show a2f3133f4b
and that made 'bc' available, in theory.

Of course, for each of my debian chroots, I had to do the
obvious
  # apt-get install bc
but that was easy enough. OTOH, for cygwin, rerunning
'install_cygwin.bat' isn't as easy. That batch file, like
'install_msw.sh', is designed to install a complete new
installation reliably. But it's not designed to upgrade an
existing installation easily; it only lets you rename or
erase an existing installation, and then completely
replace it.

I imagine that with this temporary, throwaway change:

-IF EXIST C:\cygwin64_lmi\NUL GOTO FoundOldInstallation
address@hidden IF EXIST C:\cygwin64_lmi\NUL GOTO FoundOldInstallation

running 'install_cygwin.bat' would just DTRT and add 'bc'
(and 'vim'). Vadim, can you say whether that works? I know
it's possible to add specific packages by going through the
cygwin installer's ponderous GUI, but that's hard if you do
it only once every few years (and I can't even run that
installer here at all). It seems like there must be a simple
way to adapt the careful scripting in this batch file for
a trivial update.