Implement x86 ptrace(2) requests PT_{GET,SET}{FS,GS}BASE.

MFC r284918:
Add helper fill_based_sd(9).

MFC r284919:
Add x86 PT_GETFSBASE, PT_GETGSBASE machine-depended ptrace requests to
obtain the thread %fs and %gs bases. Add x86 PT_SETFSBASE and
PT_SETGSBASE requests to set the bases from debuggers. The set
requests, similarly to the sysarch({I386,AMD64}_SET_FSBASE), override
the corresponding segment registers.

MFC r284965:
Document x86 machine-specific ptrace(2) requests.

MFC r285011:
Disallow a debugger on 64bit system to set fs/gs bases of the 32bit
process beyond the end of the process address space.

MFC r285104:
Grammar and language fixes.