kdump: Add a facility to print numbers in decimal format always

To help grepping kdump output by pid or tid it makes sence to print
some numbers in decimal format always. Eg, process or thread identifier
at least, as they already printed in decimal format.
Switch to print pid/tid arguments of some Linux signal related syscalls
in decimal format.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D40099
MFC after: 1 week