Add parse_time_header(), a wrapper around parse_header_time()

Add parse_time_header(), a wrapper around parse_header_time()

... which skips the header name for the callers which means
they can't get it wrong. Previously two callers did. This could
result in 'invalid read of size X' issues in case of value-less
headers.

On the systems I tested, X was always 1 and nobody but valgrind
cared, but with different malloc() implementations Privoxy might
be less lucky and segfault.

Partially discovered with afl-fuzz.