bootchart: properly terminate string

bootchart: properly terminate string

systemd-199/src/bootchart/store.c:289: buffer_size_warning: Calling
strncpy with a maximum size argument of 256 bytes on destination array
"ps->name" of size 256 bytes might leave the destination string
unterminated.

...and indeed, the string was used as NULL-terminated later on.

pid_cmdline_strncpy is renamed to pid_cmdline_strscpy to commemorate
the fact that it *does* properly terminate the string.