Fix _Bool functions that silently discard -1 error returns

[thegushi]

Summary

cppcheck static analysis turned up two instances of the same root cause: a function declared static _Bool that does return -1; on an error path. In C99/C11, a _Bool-returning function implicitly converts any nonzero int expression to 1 (TRUE) at the return statement, so these functions actually return TRUE, not -1, defeating callers that check for a tri-state (TRUE/FALSE/-1) contract.

• dkim_sig_hdrlistok() (libopendkim/dkim.c): a DKIM_MALLOC failure is silently treated as "header list OK" instead of propagating DKIM_STAT_NORESOURCE. cppcheck confirms the caller-side dead code: dkim.c:2170: Condition 'hstatus==-1' is always false.
• dkimf_parsehandler() (opendkim/opendkim.c): an invalid value for any On-* handling directive in the config file populates the error buffer, but the config-load call sites (!dkimf_parsehandler(...)) never see the failure, so config loading proceeds as if the directive were valid.

I grepped every _Bool-returning function in the tree for return -1; inside its body and confirmed these are the only two instances of the idiom.

Changes

• Both functions changed from static _Bool to static int, keeping their existing TRUE(1)/FALSE(0)/-1 return values as literal ints.
• Updated the dkimf_parsehandler() call sites in dkimf_config_load() from !dkimf_parsehandler(...) to dkimf_parsehandler(...) != TRUE, since both FALSE and -1 need to trip the error path.
• dkim_sig_hdrlistok()'s one call site already used an int local for the result, so no caller change was needed there.

Test plan

• autoreconf -fi && ./configure && gmake builds clean
• gmake check: 174/174 tests pass