fix: create relative home path correctly

fix: create relative home path correctly

Currently, supplying a relative path via the --prefix flag to the
useradd command triggers a bug in the creation of home directories. The
code seems to unintentionally prepend a leading "/" to all paths,
quietly transforming a relative prefixed home path into an absolute
path. This can be seen in the following strace logs from running
"useradd --create-home --prefix tmp/root squat":

```
access("tmp/root//home/squat", F_OK)    = -1 ENOENT (No such file or directory)
access("/mp", F_OK)                     = 0
access("/mp/root", F_OK)                = 0
access("/mp/root/home", F_OK)           = 0
access("/mp/root/home/squat", F_OK)     = -1 ENOENT (No such file or directory)
mkdir("/mp/root/home/squat", 000)       = 0
chown("/mp/root/home/squat", 0, 0)      = 0
chmod("/mp/root/home/squat", 0755)      = 0
chown("tmp/root//home/squat", 1000, 1000) = -1 ENOENT (No such file or directory)
chmod("tmp/root//home/squat", 0700)     = -1 ENOENT (No such file or directory)
```

Note that the relative path is correctly probed in the beginning and it
is only during the recursive creation that the path is turned into an
absolute path. This invocation results in the creation of a "/mp"
hierarchy in the root of the filesystem.

Similar problems occur when using `--prefix ./tmp/root`.

This commit fixes the handling of relative paths by not assuming that
the given path is anchored with a "/".

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>