add pbs-systemd: move string and unit handling there

the systemd config/unit parsing stays in pbs for now since
that's not usually required and uses our section config
parser

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

pbs-tools/Cargo.toml

@@ -10,6 +10,7 @@ description = "common tools used throughout pbs"
 anyhow = "1.0"
 libc = "0.2"
 nix = "0.19.1"
+nom = "5.1"
 regex = "1.2"
 serde = "1.0"
 serde_json = "1.0"