OPENAFS-SA-2024-002: make VIOCGETAL consumers stay within string bounds

OPENAFS-SA-2024-002: make VIOCGETAL consumers stay within string bounds

CVE-2024-10396

After the preceding commits, the data returned by the VIOCGETAL
pioctl (a RXAFS_FetchAcl wrapper) will safely be NUL-terminated.
However, the callers that attempt to parse the ACL string make
assumptions that the returned data will be properly formatted,
and implement a "skip to next line" functionality (under various
names) that blindly increments a char* until it finds a newline
character, which can read past the end of even a properly
NUL-terminated string if there is not a newline where one is
expected.

Adjust the various "skip to next line" functionality to keep
the current string pointer at the trailing NUL if the end of the
string is reached while searching for a newline.

Reviewed-on: https://gerrit.openafs.org/15916
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit a4ecb050540528a1bff840ff08d21f99e6ef3fbf)

Change-Id: Id2d8c0164cfaa7d03a9e37b29ff58b88cf815483
Reviewed-on: https://gerrit.openafs.org/15937
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>