libquota: remove use of PATH_MAX and replace it with QUOTA_NAME_LEN

libquota: remove use of PATH_MAX and replace it with QUOTA_NAME_LEN

PATH_MAX is not portable (for example, it doesn't exist on the Hurd).
So replace it with a new define, which defines the maximum length of
the base quota name.  As it turns out, this is substantially smaller
than PATH_MAX.

Also move the definitions relating to quotaio.c from mkquota.h to
quotaio.h, as a cleanup.

Addresses-Debian-Bug: #649689

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>