nixos: add functions and documentation for escaping systemd Exec* directives

it's really easy to accidentally write the wrong systemd Exec* directive, ones
that works most of the time but fails when users include systemd metacharacters
in arguments that are interpolated into an Exec* directive. add a few functions
analogous to escapeShellArg{,s} and some documentation on how and when to use them.

1 files changed, 0 insertions, 0 deletions

diff --git a/nixos/tests/empty-file b/nixos/tests/empty-file
new file mode 100644
index 0000000000000..e69de29bb2d1d
--- /dev/null
+++ b/nixos/tests/empty-file