Security context mech oids should be static

[frozencemetery]

Per RFC 274 section 5.1:

   mech_type            Object ID, modify, optional Security mechanism
                        used.  The returned OID value will be a pointer
                        into static storage, and should be treated as
			read-only by the caller (in particular, it does
                        not need to be freed).  If not required, specify
                        NULL.

This assumption is shared by the krb5 mechglue, which assumes this parameter can be requested without needing additional memory overhead. However, this causes the following valgrind trace from t_acquire (test_0.log):

==38725== 25 (16 direct, 9 indirect) bytes in 1 blocks are definitely lost in loss record 14 of 67
==38725==    at 0x483BAE9: calloc (vg_replace_malloc.c:760)
==38725==    by 0x53E325D: gp_conv_gssx_to_oid_alloc (gp_conv.c:82)
==38725==    by 0x53E8B04: gpm_accept_sec_context (gpm_accept_sec_context.c:75)
==38725==    by 0x53EFCCD: gssi_accept_sec_context (gpp_accept_sec_context.c:101)
==38725==    by 0x488ADF2: gss_accept_sec_context (g_accept_sec_context.c:266)
==38725==    by 0x401AD8: main (t_acquire.c:83)