Allow strongSwan to be spawned as non-root user

This patch allows for giving strongSwan only the runtime capabilities it
needs, rather than full root privileges.

Adds preprocessor directives which allow strongSwan to be configured to
 1) start up as a non-root user
 2) avoid modprobe()'ing IPsec kernel modules into the kernel, which
    would normally require root or CAP_SYS_MODULE

Additionally, some small mods to charon/libstrongswan ensure that charon
fully supports starting as a non-root user.

Tested with strongSwan 5.5.3.

src/starter/starter.c

@@ -477,6 +477,7 @@ int main (int argc, char **argv)
 		}
 	}
 
+#ifndef STARTER_ALLOW_NON_ROOT
 	/* verify that we can start */
 	if (getuid() != 0)
 	{
@@ -484,6 +485,7 @@ int main (int argc, char **argv)
 		cleanup();
 		exit(LSB_RC_NOT_ALLOWED);
 	}
+#endif
 
 	if (check_pid(pid_file))
 	{
@@ -520,6 +522,7 @@ int main (int argc, char **argv)
 		exit(LSB_RC_INVALID_ARGUMENT);
 	}
 
+#ifndef SKIP_KERNEL_IPSEC_MODPROBES
 	/* determine if we have a native netkey IPsec stack */
 	if (!starter_netkey_init())
 	{
@@ -530,6 +533,7 @@ int main (int argc, char **argv)
 			DBG1(DBG_APP, "no known IPsec stack detected, ignoring!");
 		}
 	}
+#endif
 
 	last_reload = time_monotonic(NULL);