pkgtools/pkg: bring in most of pkgsrc-synth improvements

Several updates in pkgsrc-synth work are applicable to pkgtools/pkg so
let's bring them in.  Some changes:

1) fetching segfault fixed
2) change VULNXML URL from FreeBSD ports to pkgsrc version
3) Fix vulnerability report URLs (404) to valid ones (DragonFlyBSD.org)
4) Add ability to build debuggable version
5) Fix PKG_DBDIR default setting for pkg (honor mk.conf)
6) Fix another segfault on NetBSD when pkg(8) attempts to read elfhints.
  This is only valid on FreeBSD and DragonFly, so bring in upstream
  fix to block on other platforms.
7) Set as BOOTSTRAP_PKG and block USE_CWRAPPERS for bootstrap reasons
8) realign SUBST* definitions

(marino)