Add get_fs_size() support for BSDs

Add get_fs_size() support for BSDs

Add get_fs_size() for FreeBSD/DragonFlyBSD. The same code may
work on NetBSD/OpenBSD as it uses POSIX statvfs(2).

The following results verify df(1) (which also typically uses
statfs(2) or statvfs(2)) shows the same number given that df(1)
output is correct for the given filesystems.

--
 # uname
 FreeBSD
 # cat ./test2.c
 #include <stdio.h>
 #include "os/os-freebsd.h"
 int main(int argc, char *argv[]) {
         printf("%s %llu\n", argv[1], get_fs_size(argv[1]));
         return 0;
 }
 # clang -Wall -g ./test2.c -o test2
 # ./test2 /
 / 102783143936
 # df -TH /
 Filesystem   Type    Size    Used   Avail Capacity  Mounted on
 /dev/ada0p2  ufs     112G    9.5G     94G     9%    /
                      ^^^^^^^^^^^^     ^^^
                      112G - 9.5G      bfree!=bavail
                      = 102.5G         like it differs on ext[234]
--
 # uname
 DragonFly
 # cat ./test2.c
 #include <stdio.h>
 #include "os/os-dragonfly.h"
 int main(int argc, char *argv[]) {
         printf("%s %llu\n", argv[1], get_fs_size(argv[1]));
         return 0;
 }
 # gcc -Wall -g ./test2.c -o test2
 # ./test2 /
 / 450287370240
 # df -TH /
 Filesystem  Type     Size   Used  Avail Capacity  Mounted on
 ROOT        hammer   480G    30G   450G     6%    /