Makefile: remove dup .PHONY.
USAGE: comment tweak.
Make xbps-src more robust with errors in pkg funcs and exit immediately.

xbps-src now relies in GNU bash to properly detect failures in
package template functions.

Also now if msg_error() is called we are sure xbps-src won't continue
running because the parent PID will also be killed.
Merge branch 'master' of github.com:xtraeme/xbps-src

* 'master' of github.com:xtraeme/xbps-src:
  shutils/chroot: redo previous correctly.
  shutils/chroot: ignore std{err,out} when syncing repos in xbps<0.21.