debug-generator: Allow specifying name of unit-dropin credential

debug-generator: Allow specifying name of unit-dropin credential

A fixed name is too rigid, let's give users the ability to define
custom drop-in names which at the same time also allows defining
multiple dropins per unit.

We use ~ as the separator because:
- ':' is not allowed in credential names
- '=' is used to separate credential from value in mkosi's --credential
  argument.
- '-' is commonly used in filenames
- '@' already has meaning as the unit template specifier which might be
  confusing when adding dropins for template units