mkinitfs: allow specifying compression level

(and perhaps injecting more options)

the defaults are taken from before:

gzip: -9 -> -9
xz: unspecified (-1)! -> -9
zstd: -19 -> -19
lz4: --best(-12) -> -12

i intentionally left them individual to not need to do a special case() or similar handling per type, so it's easily tunable in a specific variable for those that care, and it doesn't matter for those that don't touch it.