linux-run-hooks: Use compatible hook dir names for headers packages

The hook directory for headers postinst hooks was originally /etc/kernel/header_postinst.d. When reimplementing hook invocation in linux-run-hooks, I accidentally made the directory names for headers packages start with 'headers' and not 'header', so no existing hook scripts are detected.

Reorganise the handling of package types so we specifically handle 'headers' and convert it to a prefix of 'header_'.

Also fix documentation of the package type parameter.

Closes: #1121366