skip_prefix: return a non-const pointer

The const rules in C are such that one cannot write a
function that takes a const or non-const pointer and returns
a pointer that matches the input in const-ness. Instead, you
must take a const pointer (because you are promising not to
modify it), and then either return a const pointer (which is
safer, but annoying to callers who originally had a
non-const pointer) or a non-const pointer (less safe, as you
may accidentally drop constness, but less annoying).

This is a well-known problem, and the standard string
functions like strchr take the "less annoying" approach.
Let's mimic them. Even though this is technically less safe,
skip_prefix tends to be used alongside standard string
manipulation functions already, so it is not really
introducing a new problem.
fsck: check "tagger" lines

The fsck_tag function does not check very much about tags at
all; it just makes sure that we were able to load the
pointed-to object during the parse_tag phase. This does
check some basic things (the "object" line is OK, and the
pointed-to object exists with the expected type).

We did not, however, check the "tagger" line at all; if they
exist, we should feed them to fsck_ident (and it is OK if
they do not, as early versions of git did not include them).

This patch runs through the whole tag object during
fsck_tag, similar to what we do in fsck_commit. Some of
these checks are technically redundant with just checking
that parse_tag filled in the "tag->tagged" field. However:

  1. We have to parse through those lines anyway to get to
     the tagger line, so we need to sanity check our
     parsing.

  2. We can give more specific errors (e.g., report a
     malformed "object" line).

  3. Previously we depended on implementation details of
     parse_tag for our fsck (e.g., that it would never fill
     in "tagged" if the types did not match). Now our
     exhaustive checks are in one place, which makes it
     easier to verify exactly what fsck is checking.