Fix Solaris build

Fix Solaris build

This commit fixes a bit of rot in procfs.c caused by recent changes.

Specifically, the target_ops::to_detach change to pass down 'inferior
*' missed updating a forward declation, and the change to use
scoped_fd in more places missed removing one do_cleanups call.

  src/gdb/procfs.c: In function ‘target_ops* procfs_target()’:
  src/gdb/procfs.c:167:16: error: invalid conversion from ‘void (*)(target_ops*, const char*, int)’ to ‘void (*)(target_ops*, inferior*, int)’ [-fpermissive]
     t->to_detach = procfs_detach;
		  ^
  src/gdb/procfs.c: In function ‘ssd* proc_get_LDT_entry(procinfo*, int)’:
  src/gdb/procfs.c:1624:17: error: ‘old_chain’ was not declared in this scope
      do_cleanups (old_chain);
		   ^
  src/gdb/procfs.c: At global scope:
  src/gdb/procfs.c:90:13: error: ‘void procfs_detach(target_ops*, const char*, int)’ declared ‘static’ but never defined [-Werror=unused-function]
   static void procfs_detach (struct target_ops *, const char *, int);
	       ^
  src/gdb/procfs.c:1923:1: error: ‘void procfs_detach(target_ops*, inferior*, int)’ defined but not used [-Werror=unused-function]
   procfs_detach (struct target_ops *ops, inferior *inf, int from_tty)
   ^

gdb/ChangeLog:
2018-04-12  Pedro Alves  <palves@redhat.com>

	* procfs.c (procfs_detach): Make forward declaration's prototype
	match definition's protototype.
	(proc_get_LDT_entry): Remove stale do_cleanups call.