Implement XBPS_DISTFILES_MIRROR (second try)

The idea is to remove some burden from the original download
locations for distfiles by specifying one or more mirror locations.

+ Works with http:// or ftp:// mirrors for all $XBPS_CHROOT_CMD variants
+ Works with file:// mirrors together with uchroot and proot

README.md

@@ -74,6 +74,27 @@ To enable it:
     $ cd void-packages
     $ echo XBPS_CHROOT_CMD=proot >> etc/conf
 
+### Distfiles mirror(s)
+
+In etc/conf you may optionally define a mirror or a list of mirrors to search for distfiles.
+    $ echo 'XBPS_DISTFILES_MIRROR="ftp://192.168.100.5/gentoo/distfiles"' >> etc/conf
+
+If more than one mirror is to be searched, you can either specify multiple URLs separated
+with blanks, or add to the variable like this
+    $ echo 'XBPS_DISTFILES_MIRROR+=" http://repo.voidlinux.de/distfiles"' >> etc/conf
+Make sure to put the blank after the first double quote in this case.
+
+The mirrors are searched in order for the distfiles to build a package until the
+checksum of the downloaded file matches the one specified in the template.
+
+Ultimately, if no mirror carries the distfile, or in case all downloads failed the
+checksum verification, the original download location is used.
+
+If you use `proot` or `uchroot` for your XBPS_CHROOT_CMD, you may also specify a local path
+using the `file://` prefix or simply an absolute path on your build host (e.g. /mnt/distfiles).
+Mirror locations specified this way are bind mounted inside the chroot environment
+under $XBPS_MASTERDIR and searched for distfiles just the same as remote locations.
+
 ### Quick setup in Void
 
 Clone the `void-packages` git repository, install the bootstrap packages: