parse: use "--" with systemd-escape

[daniloegea]

Strings starting with "-" will be interpreted as systemd-escape parameters.

Like in:

network:
  wifis:
    '--version':
      access-points:
        aasasd:
          password: "asdasdasdasd"

$ ./build/src/generate --root-dir /tmp/fakeroot3/
ERROR: cannot create file /run/systemd/system/netplan-wpa-systemd 252 (252.5-2ubuntu3)
+PAM +AUDIT +SELINUX +APPARMOR +IMA +SMACK +SECCOMP +GCRYPT -GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN +IPTC +KMOD +LIBCRYPTSETUP +LIBFDISK +PCRE2 -PWQUALITY +P11KIT +QRENCODE +TPM2 +BZIP2 +LZ4 +XZ +ZLIB +ZSTD -BPF_FRAMEWORK -XKBCOMMON +UTMP +SYSVINIT default-hierarchy=unified.service: Failed to create file “/tmp/fakeroot3/run/systemd/system/netplan-wpa-systemd 252 (252.5-2ubuntu3)
+PAM +AUDIT +SELINUX +APPARMOR +IMA +SMACK +SECCOMP +GCRYPT -GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN +IPTC +KMOD +LIBCRYPTSETUP +LIBFDISK +PCRE2 -PWQUALITY +P11KIT +QRENCODE +TPM2 +BZIP2 +LZ4 +XZ +ZLIB +ZSTD -BPF_FRAMEWORK -XKBCOMMON +UTMP +SYSVINIT default-hierarchy=unified.service.P99R31”: File name too long

Description

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

+1 lgtm

This is related to 56d628e (#338)