Multiuser, Capabilities and Access Controls (#54)

* basic user, group and capability housekeeping.

add usrscope for process-wise user management
implement {get,set}*{uid,euid,gid,egid,groups} syscalls
basic acl checking mechanism
move common syscall table to arch/generic.

* patch file systems with the acl checks

make sure the user scope is copied upon fork

* syscalls: chroot, fchmodat, fchownat, faccessat

enable access check in path walker
add default directory permission for ramfs and devfs

* release the dnode lock after EACCESS in path walk

make struct stats conforming to POSIX
minor tweaks stock shell environment for better look and feel

* allow specifiying access mode when creating twifs file node

twifs: refactor the interface for easy twimap/twifs export

cake: fix overflow of index guessing when freeing, causing false
positive and possibly failing double free assertion