Multiple improvements for XBPS_PREFER_BINPKG_DEPS.

- Added a new var XBPS_REPO_LIST for the conf file, to specify
  a list of remote repositories to be used in a masterdir.
- Install all required binpkgs in one pass, if that fails install
  one of them at every pass.
- Fixed xbps-src-chroot-helper to properly catch SIG{INT,QUIT}
  when -m is specified.
- Misc cosmetics improvements.

--HG--
extra : convert_revision : 0ad7d559477ccc1fc04ab09deb791343ee453de6

xbps-src/etc/xbps-src.conf.in

@@ -39,9 +39,12 @@ XBPS_COMPRESS_CMD=xz
 
 #
 # Install required build time dependencies from binary packages,
-# XBPS_PACKAGESDIR will be used in the chroot to resolve builddeps.
+# Local repository associated with the masterdir at <masterdir>/pkg-binpkgs,
+# and remote repositories specified in XBPS_REPO_LIST will be used
+# to resolve build dependencies.
 #
 #XBPS_PREFER_BINPKG_DEPS=yes
+#XBPS_REPO_LIST="http://xbps.nopcode.org/repos/stable"
 
 #
 # Build packages with your unprivileged user in the chroot thanks