zsh-completions: kernel-install - only show existing kernels for 'remove'

When we remove a kernel, we don't remove the modules, so don't look at the modules directory to find installed kernels.
sd-dhcp-client: split sd_dhcp_lease from sd_dhcp_client

This allows us users of the library to keep copies of old leases. This is
used by networkd to know what addresses to drop (if any) when the lease
expires.

In the future this may be used by DNAv4 and sd-dhcp-server.
build-sys: autogen - enable compat libs by default
man: cryptsetup-1.6.3 now allows partition device file in system mode