process-util: move fork_agent() to process-util.[ch]

It's a relatively small wrapper around safe_fork() now, hence let's move
it over, and make its signature even more alike. Also, set a different
process name for the polkit and askpw agents.

src/basic/util.h

@@ -86,8 +86,6 @@ bool kexec_loaded(void);
 
 int prot_from_flags(int flags) _const_;
 
-int fork_agent(pid_t *pid, const int except[], unsigned n_except, const char *path, ...);
-
 bool in_initrd(void);
 void in_initrd_force(bool value);