bootctl: rework to use common verbs parsing, and add searching of ESP path

bootctl: rework to use common verbs parsing, and add searching of ESP path

This rearranges bootctl a bit, so that it uses the usual verbs parsing
routines, and automatically searches the ESP in /boot, /efi or /boot/efi, thus
increasing compatibility with mainstream distros that insist on /boot/efi.

This also adds minimal support for running bootctl in a container environment:
when run inside a container verification of the ESP via raw block device
access, trusting the container manager to mount the ESP correctly. Moreover,
EFI variables are not accessed when running in the container.