Add support for Linux capabilities

Implement Linux capability support for services, allowing them to run
with minimal required privileges instead of running as root. This uses
the modern IAB (Inheritable, Ambient, Bounding) API from libcap.

doc/features.md

@@ -130,6 +130,38 @@ unnecessary overhead, which can be removed at build-time using:
     configure --enable-auto-reload
 
 
+**Linux Capabilities**
+
+Finit supports Linux capabilities, allowing services to run with minimal
+required privileges instead of running as root. This improves security by
+following the principle of least privilege.
+
+```conf
+service [2345] name:nginx \
+        www-data:www-data \
+        caps:^cap_net_bind_service \
+        /usr/sbin/nginx -g 'daemon off;'
+```
+
+In this example, nginx runs as the unprivileged `www-data` user but retains
+the ability to bind to privileged ports (80, 443) through the
+`cap_net_bind_service` capability.
+
+The `caps:` directive uses the IAB (Inheritable, Ambient, Bounding) format:
+- `^` = Ambient (recommended) - capabilities survive exec()
+- `%` = Inheritable only - requires file capabilities
+- `!` = Bounding - block from acquiring capability
+
+Multiple capabilities can be specified as comma-separated:
+
+```conf
+caps:^cap_net_raw,^cap_net_admin,!cap_sys_admin
+```
+
+See the [Linux Capabilities](config/capabilities.md) section for detailed
+information, examples, and security best practices.
+
+
 **Cgroups**
 
 Finit supports cgroups v2 and comes with the following default groups in