Fix -Wpointer-sign warnings (char * vs u_char * mismatches)

[thegushi]

Clang on FreeBSD surfaces 156 instances of -Wpointer-sign across 9 files, all variations of implicit conversion between char * and u_char * / unsigned char *.

Affected files

• libopendkim/dkim.c (many instances — string literals assigned to const u_char *)
• libopendkim/dkim-keys.c
• libopendkim/dkim-mailparse.c
• libopendkim/dkim-report.c
• libopendkim/dkim-test.c
• opendkim/opendkim.c
• opendkim/opendkim-db.c
• opendkim/opendkim-dns.c
• opendkim/opendkim-testmsg.c

Pattern

The codebase uses u_char * for DKIM/DNS data (which is correct — it's binary content) but passes these buffers to standard C string functions that expect char *, and vice versa. Clang treats char and unsigned char as distinct types and warns on implicit pointer conversions between them; gcc does not warn by default.

Notes

• gcc with -Wformat-truncation and -Wincompatible-pointer-types is being addressed in Fix compiler warnings (-Wincompatible-pointer-types, -Wformat-truncation) #360. This is a separate, clang-specific (but also standards-correct) issue.
• Fixes will likely be (char *) or (u_char *) casts at call sites, or changing buffer declarations to match the dominant usage in context. The right answer varies by site.
• dkim.c:219-235 is likely a block of string literal initializers; those may want (const u_char *) casts or the receiving type changed.
• This does not affect correctness on any current platform — char and unsigned char have the same representation and alignment — but it is noise that makes it harder to spot real warnings.