Massive update of Kerberos compatibility code
If krb5-config produces results that don't work for Kerberos probes, fall back on manual library probing rather than just failing. If KRB5_CONFIG was explicitly set in the environment, don't use a different krb5-config based on --with-krb5. If krb5-config isn't executable, don't use it. This allows one to force library probing by setting KRB5_CONFIG to point to a nonexistent file. Sanity-check the results of krb5-config before proceeding and error out in configure if they don't work. Add separate --with-krb5-lib and --with-krb5-include configure options to set the library and include directories independently, and handle lib64 systems better and more automatically. Import the new Kerberos compatibility layer from rra-c-util and supplement it with the principal manipulation functions needed here. Take advantage of it to massively simplify the code.
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