fbsd-nat: Add helper routines for register sets using PT_[G]SETREGSET.

fbsd-nat: Add helper routines for register sets using PT_[G]SETREGSET.

FreeBSD's kernel has recently added PT_GETREGSET and PT_SETREGSET
operations to fetch a register set named by an ELF note type.  These
helper routines provide helpers to check for a register set's
existence, fetch registers for a register set, and store registers to
a register set.