imgact_elf: compute auxv buffer size instead of using magic value

[trasz]

Summary

The new buffer is somewhat larger, but there should
be no functional changes.

[kib]

Note that in theory, application might replace its auxv vector with whatever values it wants. So proc_getauxv() does not limit the count to AT_COUNT, but defined somewhat arbitrary PROC_AUXV_MAX limit. You might move this define to some global place and use it instead of AT_COUNT, as a follow-up.

Anyway, this change is already an improvement, since it avoids truncating auxv for normal core dumps.

[trasz]

(Tinderboxed.)