The code was correctly selecting the mechanism to execute, but it was
improperly setting the mechanism type of the internal context when the
selected mechanism was that of an interposer and vice versa.

When an interposer is involved the internal context is that of the
interposer, so the mechanism type of the context needs to be the
interposer oid.  Conversely, when an interposer re-enters gssapi and
presents a token with a special oid, the mechanism called is the real
mechanism, and the context returned is a real mechanism context.  In
this case the mechanism type of the context needs to be that of the
real mechanism.

ticket: 7592
target_version: 1.11.2
tags: pullup

Rename the krb5int_buf_ family of functions to use the k5_ prefix for
brevity.  Reformat some k5buf implementation code to match current
practices.

Mostly this gets rid of the trailing space on line 2 after
bb76891f5386526bdf91bc790c614fc9296cb5fa.

Since there is no overlap between the clpreauth and kdcpreauth
interface declarations, there's no particular reason to combine them
into one header.  For backward compatibility and convenience, leave
behind a preauth_plugin.h which includes both.

Fix style issues in kerrs.c and errors.c.  Rename error handling
functions to use shorter k5_ prefix.  Eliminate an inoperable
krb5int_set_error() call in gic_opte_alloc and convert the other
call to use krb5_set_error_message().

Add entries to OBJS and SRCS as well as STLIBOBJS.

Use KRB5_CALLCONV at function definition as well as declaration.

Declare missing variable in _WIN32-conditional code.

ticket: 7479 (new)
tags: pullup
target_version: 1.11

An interposer mech needs to be able to handle multiple mechanisms.
When importing a mech token for a name, cred, or context, the
interposer mech needs to know the mech type of the token being
imported.  To make this work, add SPI calls which accept a mech type
argument.

[ghudson@mit.edu: Stylistic changes, commit squashing, commit message]