Implement krb5-specific extensions

[frozencemetery]

These extensions are useful as a stopgap for applications looking to move from bindings to libkrb5 to our bindings to GSSAPI, especially gss_krb5_ccache_name().

[frozencemetery]

For ease of reference, we are talking about (from gssapi_krb5.h):

• GSS_KRB5_NT_PRINCIPAL_NAME
• gss_krb5_ccache_name
• gss_krb5_copy_ccache
• gss_krb5_get_tkt_flags
• gss_krb5_set_allowable_enctypes

These are more niche:

• gss_krb5_export_lucid_sec_context
• gss_krb5_free_lucid_sec_context
• gsskrb5_extract_authz_data_from_sec_context
• gsskrb5_extract_authtime_from_sec_context

The following functionality is also available through the cred_store API
extensions so we may elect not to implement them at all:

• gss_krb5_set_cred_rcache
• gss_krb5_import_cred

[krizex]

I am also looking forward to the implementation of gss_krb5_set_allowable_enctypes which could help me get rid of the krb5.conf when using this library.

[jborean93]

#261 implements the following:

• GSS_KRB5_NT_PRINCIPAL_NAME
• gss_krb5_ccache_name
• gss_krb5_get_tkt_flags
• gss_krb5_set_allowable_enctypes
• gss_krb5_export_lucid_sec_context
• gss_krb5_free_lucid_sec_context
• gsskrb5_extract_authz_data_from_sec_context
• gsskrb5_extract_authtime_from_sec_context
• gss_krb5_import_cred
  • There is a note about this being available through the cred_store API. Unfortunately Heimdal did not implement gss_acquire_cred_from making this one way of importing a CCACHE for Heimdal on older versions - important for a use case I have

I didn't implement the following:

• gss_krb5_set_cred_rcache
  • Not present on Heimdal - seems quite limited to just implement for one
• gss_krb5_copy_ccache
  • It is marked as deprecated on macOS Heimdal implementation (not Heimdal itself)
  • It will compile but it does emit a warning and honestly just seems more trouble than it's worth