Include file ccache name in error messages

When a FILE ccache method returns an error, append the filename to the
standard message for the code.  Remove code to set extended messages
in helper functions as they would just be overwritten.

Also change the interpretation of errno values.  Treat ENAMETOOLONG as
KRB5_FCC_NOFILE instead of KRB5_FCC_INTERNAL, since it has an external
cause and a name that long can't be opened by normal means.  Treat
EROFS as KRB5_FCC_PERM.  Treat ENOTDIR and ELOOP as KRB5_FCC_NOFILE
instead of KRB5_FCC_PERM as both errors imply that the full pathname
doesn't exist.  Treat EBUSY and ETXTBSY as KRB5_CC_IO instead of
KRB5_FCC_PERM as they indicate a conflict rather than a permission
issue.

[ghudson@mit.edu: renamed set_error to set_errmsg_filename; removed
now-inoperative code to set extended messages in helper functions;
trimmed changes to interpret_errno; clarified and shortened commit
message]

ticket: 8052 (new)