libthread_db: unsign map_len and use reallocarray(3).

Lengths are not negative, so map_len should be unsigned. Unsign the
corresponding indexes too and bring a small use of reallocarray(3).

Reorder the memset to be consistent with the reallocarray.

  r315900:
    Cleanup structures related to VFP and/or mcontext_t.  - in mcontext_t,
    rename newer used 'union __vfp' to equaly sized 'mc_spare'.
      Space allocated by 'union __vfp' is too small and cannot hold full VFP
      context.
    - move structures defined in fp.h to more appropriate headers.  - remove
    all unused VFP structures.
  r315973:
    Save VFP state on fork().  Update the copy of VFP state in PCB before it is
    cloned for new process.
  r315974:
    Preserve VFP state across signal delivery.

r312452-r312512:

- Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones

  This simplifies pathing in make/displayed output

- Use .CURDIR:H instead of .CURDIR to simplify pathing in output, etc

Sponsored by:	The FreeBSD Foundation

Sponsored by:	DARPA, AFRL
Sponsored by:	HEIF5
Differential Revision:	https://reviews.freebsd.org/D5064

This both avoids some dependencies on xinstall.host and allows
bootstrapping on older releases to work due to lack of at least 'install -l'
support.

Sponsored by:	EMC / Isilon Storage Division

Off by default, build behaves normally.
WITH_META_MODE we get auto objdir creation, the ability to
start build from anywhere in the tree.

Still need to add real targets under targets/ to build packages.

Differential Revision:       D2796
Reviewed by: brooks imp