Fix gss_str_to_oid and gss_oid_to_str edge cases

Neither function correctly handled OIDs whose second arc exceeds 47
(theoretically possible if the first arc is 2).  gss_str_to_oid had
additional problems: it used scanf, it didn't consistently protect
against read overrun if the input buffer wasn't null-terminated, and
it could get confused by + or - characters in the first two arcs.  Fix
gss_oid_to_str and rewrite gss_str_to_oid.

Also add a test program.

https://github.com/krb5/krb5/commit/9b702abe222d4b279d5869f96f09074452478b1e
Author: Greg Hudson <ghudson@mit.edu>
Commit: 9b702abe222d4b279d5869f96f09074452478b1e
Branch: master
 .gitignore                       |    1 +
 src/lib/gssapi/generic/oid_ops.c |  246 +++++++++++++++++++-------------------
 src/tests/gssapi/Makefile.in     |    9 +-
 src/tests/gssapi/t_oid.c         |  221 ++++++++++++++++++++++++++++++++++
 4 files changed, 350 insertions(+), 127 deletions(-)