savecore: decrease filename buffer sizes

[mhorne]

Summary

All files are now created relative to savedirfd, e.g. with openat(2).
Therefore, we do not need character buffers to be PATH_MAX bytes long,
just long enough to hold the complete filename. 32 bytes is long enough
in all cases. These can be allocated on the stack.

While here, fix an error message that attempts to use an uninitialized
infoname.

[mhorne]

sbin/savecore/savecore.c

700–702

Choosing to list the static var first.

[markj]

sbin/savecore/savecore.c

700–702

I prefer that too.

703

The keyname declaration fits on the previous line now.