common: auto-register api_fs_umount EXIT trap

This corrects some weird/wrong EXIT trap decls in arch-chroot and
pacstrap that should have been touched as part of the cleanup in
517fcff074.

arch-chroot.in

@@ -26,8 +26,6 @@ shift
 
 [[ -d $chrootdir ]] || die "Can't create chroot on non-directory %s" "$chrootdir"
 
-trap '{ api_fs_umount "$chrootdir"; umount "$chrootdir/etc/resolv.conf"; } 2>/dev/null' EXIT
-
 api_fs_mount "$chrootdir" || die "failed to setup API filesystems in chroot %s" "$chrootdir"
 track_mount /etc/resolv.conf "$chrootdir/etc/resolv.conf" --bind
 

common

@@ -17,6 +17,7 @@ track_mount() {
 
 api_fs_mount() {
   CHROOT_ACTIVE_MOUNTS=()
+  trap 'api_fs_umount' EXIT
   { mountpoint -q "$1" || track_mount "$1" "$1" --bind; } &&
   track_mount proc "$1/proc" -t proc -o nosuid,noexec,nodev &&
   track_mount sys "$1/sys" -t sysfs -o nosuid,noexec,nodev &&

pacstrap.in

@@ -101,9 +101,6 @@ mkdir -m 0755 -p "$newroot"/var/{cache/pacman/pkg,lib/pacman,log} "$newroot"/{de
 mkdir -m 1777 -p "$newroot"/tmp
 mkdir -m 0555 -p "$newroot"/{sys,proc}
 
-# always call umount on quit after this point
-trap 'api_fs_umount "$newroot" 2>/dev/null' EXIT
-
 # mount API filesystems
 api_fs_mount "$newroot" || die "failed to setup API filesystems in new root"