jam: jcache: typo in comment
jam: delete_jamfile_cache(): don't free cache_file

The value is initialized from a static variable returned by
jcache_name().
jam: get_jcache(): clean up when reading the cache failed

When reading the cache file failed we have to assume it is corrupt and
we should use any entries read from it. So now we remove the ones we
read again.
jam: jcache: read_file(): fail when encountering too long lines

For its input files jam uses a buffer of 512 chars for fgets(). Lines
would therefore be split silently after that length, which could lead to
"interesting" issues. Now we fail to prevent the situation from going
unnoticed.