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.

src/svc.h

@@ -191,6 +191,7 @@ typedef struct svc {
 	/* Identity */
 	char	       username[MAX_USER_LEN];
 	char	       group[MAX_USER_LEN];
+	char	       capabilities[MAX_CMD_LEN];
 
 	/* Command, arguments and service description */
 	char	       cmd[MAX_CMD_LEN];