Add `/sigfile <path>` parameter to specify custom signature file location.

[piotrkochan]

Adds /sigfile <path> parameter to specify custom signature file location.

• Maintains backward compatibility
• Falls back to default behavior if custom file unavailable
• Updated help text

Usage: photorec /sigfile custom.sig [options] [device]

[piotrkochan]

I probably should move static char *custom_sig_file = NULL; to struct ph_param. What do you think, @cgsecurity?

[cgsecurity]

I probably should move static char *custom_sig_file = NULL; to struct ph_param. What do you think, @cgsecurity?

As it's only used once, I think it's OK as a "static char*".

As the function set_custom_signature_file() is defined in file_sig.c and not filegen.c, it may be better to define its prototype in a new file file_sig.h instead of filegen.h.