Consolidate x86 debug register code for BSD native targets.
    
    Move the debug register support code from amd64bsd-nat.c and
    i386bsd-nat.c into a shared x86bsd-nat.c.
    
    Instead of setting up x86_dr_low in amd64fbsd-nat.c and
    i386fbsd-nat.c, add a x86bsd_target function that creates a new target
    that inherits from inf_ptrace and sets up x86 debug registers if
    supported.  In addition to initializing x86_dr_low, the x86bsd target
    installs a custom mourn_inferior target operation to clean up the
    x86 debug register state.  Previously this was only done on amd64.
    Now it will be done for both i386 and amd64.  The i386bsd_target and
    amd64bsd_target functions create targets that inherit from x86bsd
    rather than inf_ptrace.
    
    gdb/ChangeLog:
    
    	* Makefile.in [HFILES_NO_SRCDIR]: Replace 'amd64bsd-nat.h' with
    	'x86bsd-nat.h'.
    	* amd64bsd-nat.c: Include 'x86bsd-nat.h' instead of
    	'amd64bsd-nat.h'.
    	(amd64bsd_xsave_len): Rename and move to x86bsd-nat.c.
    	(amd64bsd_fetch_inferior_registers): Replace 'amd64bsd_xsave_len'
    	with 'x86bsd_xsave_len'.
    	(amd64bsd_store_inferior_registers): Likewise.
    	(amd64bsd_target): Inherit from x86bsd_target.
    	(amd64bsd_dr_get): Rename and move to x86bsd-nat.c.
    	(amd64bsd_dr_set): Likewise.
    	(amd64bsd_dr_set_control): Likewise.
    	(amd64bsd_dr_set_addr): Likewise.
    	(amd64bsd_dr_get_addr): Likewise.
    	(amd64bsd_dr_get_status): Likewise.
    	(amd64bsd_dr_get_control): Likewise.
    	* amd64fbsd-nat.c: Include 'x86bsd-nat.h' instead of
    	'amd64bsd-nat.h'.
    	(super_mourn_inferior): Move to x86bsd-nat.c.
    	(amd64fbsd_mourn_inferior): Rename and move to x86bsd-nat.c.
    	(amd64fbsd_read_description): Replace 'amd64bsd_xsave_len' with
    	'x86bsd_xsave_len'.
    	(_initialize_amd64fbsd_nat): Remove x86 watchpoint setup and
    	mourn_inferior' target op.
    	* config/i386/fbsd.mh (NATDEPFILES): Add x86bsd-nat.o.
    	* config/i386/fbsd64.mh: Likewise.
    	* config/i386/nbsd64.mh: Likewise.
    	* config/i386/nbsdelf.mh: Likewise.
    	* config/i386/obsd.mh: Likewise.
    	* config/i386/obsd64.mh: Likewise.
    	* i386bsd-nat.c: Include 'x86bsd-nat.h'.
    	(i386bsd_xsave_len): Rename and move to x86bsd-nat.c.
    	(i386bsd_fetch_inferior_registers): Replace 'i386bsd_xsave_len'
    	with 'x86bsd_xsave_len'.
    	(i386bsd_store_inferior_registers): Likewise.
    	(i386bsd_target): Inherit from x86bsd_target.
    	(i386bsd_dr_get): Rename and move to x86bsd-nat.c.
    	(i386bsd_dr_set): Likewise.
    	(i386bsd_dr_set_control): Likewise.
    	(i386bsd_dr_set_addr): Likewise.
    	(i386bsd_dr_get_addr): Likewise.
    	(i386bsd_dr_get_status): Likewise.
    	(i386bsd_dr_get_control): Likewise.
    	* i386bsd-nat.h (i386bsd_xsave_len): Remove.
    	(i386bsd_dr_set_control): Remove.
    	(i386bsd_dr_set_addr): Remove.
    	(i386bsd_dr_get_addr): Remove.
    	(i386bsd_dr_get_status): Remove.
    	(i386bsd_dr_get_control): Remove.
    	* i386fbsd-nat.c: Include 'x86bsd-nat.h'.
    	(i386fbsd_read_description): Replace 'i386bsd_xsave_len' with
    	'x86bsd_xsave_len'.
    	(_initialize_i386fbsd_nat): Remove x86 watchpoint setup and
    	mourn_inferior' target op.
    	* x86bsd-nat.c: New file.
    	* x86bsd-nat.h: New file.