exec: Introduce osmo_system_nowait2() to allow specify a user

For a process running as root, it may be desirable to drop privileges
down to a normal user  before executing an external command.  Let's
add a new API function for that.

Change-Id: If1431f930f72a8d6c1d102426874a11b7a2debd9
diff --git a/include/osmocom/core/exec.h b/include/osmocom/core/exec.h
index 6bbd352..e63ec11 100644
--- a/include/osmocom/core/exec.h
+++ b/include/osmocom/core/exec.h
@@ -25,4 +25,5 @@
 int osmo_environment_filter(char **out, size_t out_len, char **in, const char **whitelist);
 int osmo_environment_append(char **out, size_t out_len, char **in);
 int osmo_close_all_fds_above(int last_fd_to_keep);
+int osmo_system_nowait2(const char *command, const char **env_whitelist, char **addl_env, const char *user);
 int osmo_system_nowait(const char *command, const char **env_whitelist, char **addl_env);