fbsd nat: Disable address space randomization when requested.

fbsd nat: Disable address space randomization when requested.

Use procctl(2) with PROC_ASLR_CTL to disable address space
randomization in the current gdb process before forking a child
process for a new inferior when address space randomization is
disabled.

gdb/ChangeLog:

	* configure.ac: Check for <sys/procctl.h>.
	* config.in, configure: Regenerate.
	* fbsd-nat.c: Include <sys/procctl.h> if present.
	[PROC_ASLR_CTL] (maybe_disable_address_space_randomization): New.
	(fbsd_nat_target::create_inferior)
	(fbsd_nat_target::supports_disable_randomization): New.
	* fbsd-nat.h (fbsd_nat_target::create_inferior)
	(fbsd_nat_target::supports_disable_randomization): New.