Add support for COMPAT6 syscalls.
Add support for COMPAT6 syscalls.

Also, change the visibility of compat syscalls a slightly.  Compat
syscalls were missing from 'syscalls.h' entirely.  This additionally adds
them with their compat prefix.  eg: SYS_freebsd6_mmap.

Also, the syscalls.c names strings have different prefixes to differentiate
syscalls. Instead of several "old.mmap" strings, there will now be a
"compat.mmap" and "compat6.mmap" etc.  Before, both would have had the
same "old.mmap" label.

Approved by:  re

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase