This commit changes the add_user_thunk to point to a new routine,
initialize_user_cpu, which sets all the registers that were previously
unset when starting a new user thread. The values for rdi and rsi are
popped off the initial stack values that add_user_thunk sets up, so that
user thread procs can take up to two arguments.
To suppor this, j6_thread_create gained two new arguments, which are
passed on to the thread.
This also let me finally get rid of the hack of passing an argument in
rsp when starting init.
This commit re-adds testapp to the default manifest and does some
housecleaning on the module:
- Remove the old serial.* and io.*
- Update it to use current syscall APIs
- Update it to use libj6's higher-level thread API
This new class makes it easier for user programs to spawn threads. This
change also includes support for .hh files in modules, to differentiate
headers that are C++-only in system libraries.
