BUG/MINOR: cache: Fix copy of value when parsing maxage

BUG/MINOR: cache: Fix copy of value when parsing maxage

During maxage parsing, the size of the value was not properly computed when
it was copied into the trash chunk. The name (max-age or s-maxage) must be
skipped with the '=' character. But instead of doing a subtraction, and
addition was performed, adding 2 extra bytes to the value used for the
convertion to integer.

In addition, the "chunk_memcat(chk, "", 1)" operation to add a trailing
NULL-byte was replaced by "*(b_tail(chk)) = '\0'". It a bit easier to
understand.

This patch should be backported to all stable versions.