Various gssalloc fixes

Various gssalloc fixes

The DEBUG_GSSALLOC version of gssalloc_realloc() must add the sentinel
size to the byte count.

The mechglue gss_decapsulate_token(), gss_encapsulate_token(), and
gss_export_sec_context() must use gssalloc_malloc() to allocate
output buffers.

The krb5 mech's gss_export_name_composite() and gss_pseudo_random()
implementations must use gssalloc_malloc() to allocate output buffers.

SPNEGO's gss_display_status() implementation must use gssalloc for the
output buffer.

The sample GSS server must use gss_release_buffer() to free the result
of gss_export_sec_context().

ticket: 8852 (new)
tags: pullup
target_version: 1.17-next
target_version: 1.16-next