kernel-install: rewrite in C

kernel-install: rewrite in C

This is mostly a one-to-one translation of kernel-install.sh, except for
the followings:
- BOOT_ROOT is searched with find_{esp,xbootldr}_and_warn().
- entry token is searched with boot_entry_token_ensure().
- inspect command verboses more information, e.g. found plugins,
  environment variables explicitly passed to plugins, arguments passed
  to plugins.
- paths specified in $KERNEL_INSTALL_PLUGINS must be absolute.
- LC_COLLATE is set to C.UTF-8 (or any specified on build time).

By writing kernel-install C, we can share the code used by bootctl or
so, and can introduce --root and/or --image options later.