X86PagingStructuresPAE: Zero fPageDirPointerTable in constructor

... and use it as a guard in the destructor. Fixes crash when running
out of memory and Init() is not called.
useradd: Improve usage text
getgrgid_r()/getgrname_r(): Fix group not found return value
getpw{nam,uid}[_r]: Fix return value behavior

... when the user is not found.
getgr{nam,gid}[_r](): Fix retrieving group members
useradd: Support specifying group by name
Add userdel
useradd: small style fix
useradd: Create the new user with a locked password
passwd: Add option -d to delete a user's password
Enable -Werror in src/bin/multiuser
Add group{add,del,mod}