Support -ddump-signatures in EPS backend

This option is required to detect differences in multi-page regression tests.

Override the option to disable it for input/regression/font-name-add-files.ly because it doesn't work there due to unusual font handling.

I tested it by introducing differences into the musical systems in these test cases:

• page-turn-page-breaking-repeats
• repeat-volta

It detected the differences in both cases.

I also tried changing some titles in baerenreiter-sarabande, and they were not detected, so there is clearly room for improvement.