Move fork_agent() into shared/

Currently it's only used in two places in src/shared/, so the function was
already included just once in compiled code. But it seems appropriate to
move it there anyway, because library code should have no need to fork
agents, so it doesn't belong in basic/.

src/shared/exec-util.h

@@ -48,3 +48,5 @@ const char* exec_command_flags_to_string(ExecCommandFlags i);
 ExecCommandFlags exec_command_flags_from_string(const char *s);
 
 int fexecve_or_execve(int executable_fd, const char *executable, char *const argv[], char *const envp[]);
+
+int fork_agent(const char *name, int except[], size_t n_except, pid_t *ret_pid, const char *path, ...) _sentinel_;