Convert oslfsck to lopsub.

Convert oslfsck to lopsub.

The gengetopt project seems to be dead (last release was five years
ago), so we eventually need to switch to something else.

This commit migrates the option parser of oslfsck to the lopsub
library, which turned out to be rather simple. As a result, the osl
package no longer depends on gengetopt and help2man.

The change only affects the oslfsck executable. No changes to the
library proper.