Warn if /proc is not accessible

Warn if /proc is not accessible

There's a buildroot where I want to debug a binary, and I tried to
connect to it from outside, but got very weird errors like
architecture mismatch or protocol errors.  At last, after switching on
'--debug' for gdbserver I found a message 'Can't open /proc/pid/'
message and suddenly found that I forgot to mount procfs in my
buildroot.

Make discovering the problem easier by making GDB / GDBserver warn
(even without --debug) if /proc can not be accessed.

Native debugging:

 (gdb) start
 Temporary breakpoint 1 at 0x400835: file test.c, line 10.
 Starting program: /tmp/test
 warning: /proc is not accessible.

GDBserver/remote debugging:

 $ ./gdbserver :9999 ./gdbserver
 gdbserver: /proc is not accessible.

gdb/ChangeLog:
2018-07-04  Vyacheslav Barinov  <v.barinov@samsung.com>
	    Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_init_ptrace): Rename to ...
	(linux_init_ptrace_procfs): ... this.  Call
	linux_proc_init_warnings.
	(linux_nat_target::post_attach)
	(linux_nat_target::post_startup_inferior): Adjust.
	* nat/linux-procfs.c (linux_proc_init_warnings): Define function.
	* nat/linux-procfs.h (linux_proc_init_warnings): Declare function.

gdb/gdbserver/ChangeLog:
2018-07-04  Vyacheslav Barinov  <v.barinov@samsung.com>
	    Pedro Alves  <palves@redhat.com>

	* linux-low.c (initialize_low): Call linux_proc_init_warnings.