xbps-src: introduce support for multiple chroot implementations.

Available implementations at common/chroot-style/*.sh. Each .sh
script there implements a chroot style to be able to chroot and bind
mount with multiple utilities. The current supported list:

 - uunshare (uses xbps-uunshare(8))
 - uchroot (uses xbps-uchroot(8))
 - proot (uses proot, see http://proot.me)

The XBPS_CHROOT_CMD can be set in etc/conf to use a specific implementation,
and XBPS_CHROOT_CMD_ARGS to pass in additional arguments to the cmd.

common/chroot-style/uchroot.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+#
+# This chroot script uses xbps-uchroot(8).
+#
+readonly MASTERDIR="$1"
+readonly DISTDIR="$2"
+readonly HOSTDIR="$3"
+readonly EXTRA_ARGS="$4"
+readonly COMMAND="$5"
+shift 5
+readonly COMMAND_ARGS="$@"
+
+if ! command -v xbps-uchroot >/dev/null 2>&1; then
+	exit 1
+fi
+
+if [ -z "$MASTERDIR" -o -z "$DISTDIR" -o -z "$COMMAND" ]; then
+	echo "$0 MASTERDIR/DISTDIR/COMMAND not set"
+	exit 1
+fi
+
+exec xbps-uchroot $EXTRA_ARGS -D $DISTDIR ${HOSTDIR:+-H $HOSTDIR} $XBPS_MASTERDIR $COMMAND $COMMAND_ARGS