Link modules to DYN type

[Zhenlei Huang]

Hi,

I'm recently working on https://reviews.freebsd.org/D39638 (sysctl(9): Enable 
vnet sysctl variables be loader tunable),
the changes to `sys/kern/link_elf_obj.c` are runtime tested, but not those to 
`sys/kern/link_elf.c` .

After some hacking I realized that `link_elf.c` is for EXEC (Executable file) 
or DYN (Shared object file), and `link_elf_obj.c` is
for REL (Relocatable file).

```
/* link_elf.c */
static int
link_elf_load_file(linker_class_t cls, const char* filename,
    linker_file_t* result)
{
...
        if (hdr->e_type != ET_EXEC && hdr->e_type != ET_DYN) {
                error = ENOSYS;
                goto out;
        }
...
}


/* link_elf_obj.c */
static int
link_elf_load_file(linker_class_t cls, const char *filename,
    linker_file_t *result)
{
...
        if (hdr->e_type != ET_REL) {
                error = ENOSYS;
                goto out;
        }
...
}
```

Run the following snip:
```
# find /boot/kernel -type f -name "*.ko" -exec readelf -h {} \; | grep Type
```
shows that all the kernel modules' types are `REL (Relocatable file)`.

I guess if some module such as if_bridge is linked to DYN type, then I can do 
runtime for the changes to `sys/kern/link_elf.c`.

I'm not familiar with elf and linkers, is that ( compile module and link it to 
DYN type ) possible ?


Best regards,
Zhenlei