Fix AT_EXECPATH for direct exec mode.

[kib]

Summary

The somewhat problematic case is when the executable path is relative and either $PATH use is not enabled or it contains '/' so $PATH search is not performed. In this case AT_EXECPATH is relative, I might fix this later.

[rstone]

libexec/rtld-elf/rtld.c

510

Testing has shown that aux_info[AT_EXECPATH] is not guaranteed to be non-NULL

[kib]

Handle a case when AT_EXECPATH is not present in auxv.
Add XXX comments.

[kib]

Remove pointless reassignment.