Constify target_pid_to_exec_file

This changes target_pid_to_exec_file and target_ops::pid_to_exec_file
to return a "const char *".  I couldn't build many of these targets,
but did examine the code by hand -- also, as this only affects the
return type, it's normally pretty safe.  This brings gdb and gdbserver
a bit closer, and allows for the removal of a const_cast as well.

gdb/fbsd-nat.c

@@ -57,7 +57,7 @@
 /* Return the name of a file that can be opened to get the symbols for
    the child process identified by PID.  */
 
-char *
+const char *
 fbsd_nat_target::pid_to_exec_file (int pid)
 {
   static char buf[PATH_MAX];