support PTRACE_SETREGS.
In struct process, add 'unsigned long *ptrace_debugreg', instead of 'struct user *userp'.
debug registers are read/written from/to ptrace_debugreg, save/restore in schedule().
most general registers are proc->uctx.
fs_base is proc->thread.tlsblock_base.
gs_base,ds,es,fs,gs and orig_rax are uncompleted.
other members in 'struct user' are ignored, same as Linux implementation.
refs #257
refs #373
refs #263
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