vma-iter: Improvements for Linux and BSD platforms. - Add support for DragonFly BSD. - Make it more reliable on Linux, GNU/kFreeBSD, FreeBSD, NetBSD. * lib/vma-iter.c (struct rofile, rof_open, rof_peekchar, rof_close): Read the entire file into memory in a single system call. (vma_iterate): Update. Read from /proc on DragonFly BSD like on FreeBSD. * lib/vma-iter.h (VMA_ITERATE_SUPPORTED): Define also on DragonFly BSD.