makechrootpkg: Have functions be more function-y.

Rather than them simply being named blocks of code with braces around
them.

That is: have them take things via arguments rather than global
variables.

Specific notes:

 - create_chroot->sync_chroot:

   I pulled out locking the destination chroot; getting that lock is
   now the caller's responsibility.  It still handles locking the
   source chroot though.

   I pulled the `if [[ ! -d $copydir ]] || $clean_first;` check out; it is
   now the caller's responsibility to use that check when deciding if to
   call sync_chroot.

   However, when pulling that check out, I left it as `if true;`, to
   keep an indentation level.  This patch has had to be rebased/merged
   many times, and changing the indentation is a sure way to make that
   go less smoothly; I'm not going to re-indent this block until I see
   the check removed in the git.archlinux.org/devtools.git repository.

 - install_packages:

    1. Receive the list of packages as arguments, rather than a global
       variable.
    2. Make the caller responsible for looking at PKGBUILD.  From the
       name and arguments, one would never expect it to look at PKGBUILD.