Added support to download all dependencies even
if the euid does not have write perms to rootdir.
In this mode we only care if cachedir is writable,
rootdir access is not necessary.
This is really useful to download all binary packages
required by any number of packages as any regular
user to later perform off-line installations, i.e:
```
$ xbps-install -c $PWD/cachedir -yD xbps
...
$ tree cachedir
cachedir/
├── acl-2.2.53_1.x86_64-musl.xbps
├── acl-2.2.53_1.x86_64-musl.xbps.sig
├── attr-2.4.48_1.x86_64-musl.xbps
├── attr-2.4.48_1.x86_64-musl.xbps.sig
├── bzip2-1.0.8_1.x86_64-musl.xbps
├── bzip2-1.0.8_1.x86_64-musl.xbps.sig
├── ca-certificates-20190110_1.noarch.xbps
├── ca-certificates-20190110_1.noarch.xbps.sig
├── libarchive-3.4.1_1.x86_64-musl.xbps
├── libarchive-3.4.1_1.x86_64-musl.xbps.sig
├── libcrypto45-3.0.2_2.x86_64-musl.xbps
├── libcrypto45-3.0.2_2.x86_64-musl.xbps.sig
├── liblz4-1.9.2_1.x86_64-musl.xbps
├── liblz4-1.9.2_1.x86_64-musl.xbps.sig
├── liblzma-5.2.4_2.x86_64-musl.xbps
├── liblzma-5.2.4_2.x86_64-musl.xbps.sig
├── libressl-3.0.2_2.x86_64-musl.xbps
├── libressl-3.0.2_2.x86_64-musl.xbps.sig
├── libssl47-3.0.2_2.x86_64-musl.xbps
├── libssl47-3.0.2_2.x86_64-musl.xbps.sig
├── libtls19-3.0.2_2.x86_64-musl.xbps
├── libtls19-3.0.2_2.x86_64-musl.xbps.sig
├── libxbps-0.57.1_8.x86_64-musl.xbps
├── libxbps-0.57.1_8.x86_64-musl.xbps.sig
├── libzstd-1.4.4_1.x86_64-musl.xbps
├── libzstd-1.4.4_1.x86_64-musl.xbps.sig
├── musl-1.1.24_1.x86_64-musl.xbps
├── musl-1.1.24_1.x86_64-musl.xbps.sig
├── run-parts-4.9.1_1.x86_64-musl.xbps
├── run-parts-4.9.1_1.x86_64-musl.xbps.sig
├── xbps-0.57.1_8.x86_64-musl.xbps
├── xbps-0.57.1_8.x86_64-musl.xbps.sig
├── xbps-triggers-0.113_3.noarch.xbps
├── xbps-triggers-0.113_3.noarch.xbps.sig
├── zlib-1.2.11_3.x86_64-musl.xbps
└── zlib-1.2.11_3.x86_64-musl.xbps.sig
0 directories, 36 files
$
```
Inpired by #213Closes#213
