Add <~cond> condition modifier for tightly coupled services

Conditions in Finit are dependencies: if A is asserted, service B is
allowed to run.  When A goes through FLUX (e.g., upstream reloads),
dependents are PAUSED and then simply resumed when the condition is
reasserted -- this is the correct behavior for barrier-style deps
like <pid/syslogd>.

However, some setups have tightly coupled services where dependents
must be reloaded/restarted when an upstream service reloads, not just
resumed.  E.g., the FRR routing stack on Infix OS:

    netd <pid/mgmtd> ← zebra <!pid/netd> ← {staticd,ripd} <!pid/zebra>

When netd reloads (SIGHUP), zebra and its dependents must be restarted
to pick up the new configuration.

The new '~' condition prefix marks a dependency as flux-sensitive:

    service <!~pid/netd> name:zebra ...

When the upstream condition goes FLUX and returns to ON, the dependent
is reloaded (SIGHUP) or restarted (noreload '!') instead of merely
resumed.  Transitivity follows naturally through the condition chain.

Closes #416
Closes #476

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/config/services.md

@@ -47,6 +47,14 @@ waits for another service, `zebra`, to have created its PID file in
 *all* files in `/var/run`, for each file named `*.pid`, or `*/pid`,
 Finit opens it and find the matching `NAME:ID` using the PID.
 
+The condition can be prefixed with `!` and/or `~`:
+
+ - `<!pid/zebra>` -- `ospfd` does not support `SIGHUP` (noreload)
+ - `<~pid/zebra>` -- propagate reload from `zebra` to `ospfd`
+ - `<!~pid/zebra>` -- both: noreload and propagate reload
+
+For details, see the [Finit Conditions](../conditions.md) document.
+
 Some services do not maintain a PID file and rather than patching each
 application Finit provides a workaround.  A `pid` keyword can be set
 to have Finit automatically create (when starting) and later remove