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/basic/process-util.h

@@ -174,8 +174,6 @@ static inline int safe_fork(const char *name, ForkFlags flags, pid_t *ret_pid) {
 
 int namespace_fork(const char *outer_name, const char *inner_name, int except_fds[], size_t n_except_fds, ForkFlags flags, int pidns_fd, int mntns_fd, int netns_fd, int userns_fd, int root_fd, pid_t *ret_pid);
 
-int fork_agent(const char *name, int except[], size_t n_except, pid_t *pid, const char *path, ...) _sentinel_;
-
 int set_oom_score_adjust(int value);
 
 /* The highest possibly (theoretic) pid_t value on this architecture. */