libkmod/libkmod-signature: rework struct kmod_signature_info

libkmod/libkmod-signature: rework struct kmod_signature_info

Currently, we use a stack allocated instance which bolts of misc private
data via a void *, while also having an optional free callback.

In kmod we opt to pre-calculate the total size, do a one-off allocation,
copy the data as needed, adjusting the pointers.

Doing the same here, gives us a mixed bag of benefits:
 - shorter and simpler code
 - smaller binary - ~100 bytes off
 - fewer instructions - ~40 per module
 - few allocations - ~2 per module
 - extra bytes are allocated - ~180 per module

The updated code seems far more natural and consistent with the
code-base. Although, if we really want to squeeze more cycles we could
use a reasonably large stack buffer and fallback to heap.

v2:
 - don't leak on d2i_PKCS7_bio failure
 - use +1 (instead of sizeof('\0')) for the null terminator

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/427
Signed-off-by: Lucas De Marchi <demarchi@kernel.org>