vmstat: add sigstat support for Linux

This adds support to process /proc signal information and make sense of it all.

Note: this v1 goes acknowledging calling $(vmstat --sigstat 1) doesn't work yet, and neither does $(vmstat -g), however this can be fixed. I am also slightly aware perhaps proc/ isn't the right place for this so looking for guidance. I also realize we have a few helpers for signal names, I just didn't feel that this Linux specific solution is yet prime-ready for sharing it with those other Linux calls given it is relying on an undocumented libc helper, sys_sigabbrev.

Figured we'd get the reviews going now.

Signed-off-by: Luis Chamberlain mcgrof@kernel.org