MFC 299458: Fix buffer overrun in gcore(1) NT_PRPSINFO

MFC 299458: Fix buffer overrun in gcore(1) NT_PRPSINFO

Use size of destination buffer, rather than a constant that may or may not
correspond to the source buffer, to restrict the length of copied strings.  In
particular, pr_fname has 16+1 characters but MAXCOMLEN is 18+1.

Use strlcpy instead of strncpy to ensure the result is nul-terminated.  This
seems to be what is expected of these fields.

git-svn-id: svn://svn.freebsd.org/base/stable/10@306781 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f