[krbdev.mit.edu #8566] krb5_init_context() should detect set-uid-ness

[Nico Williams via RT]

https://github.com/heimdal/heimdal/pull/270

Adds more intelligence to issuid():

 - if AT_EXECPATH/AT_EXECFN/AT_SUN_EXECNAME is set, it will stat() that
   to see if that file is set-uid/set-gid

 - it will more intelligently detect getauxval() implementations that do
   not set errno = ENOENT when the type is not found and will try
   reading /proc/self/auxv if it is available

 - it will use getresuid() and getregid() if available as part of the
   last ditch effort (that now should only apply to non-ELF programs)

 - it will memoize its results to avoid doing slow things repeatedly