Fix regression: cannot start with LD_PRELOAD=libSegFault.so (PR gdb/18653#c7)

Fix regression: cannot start with LD_PRELOAD=libSegFault.so (PR gdb/18653#c7)

At https://sourceware.org/bugzilla/show_bug.cgi?id=18653#c7, Andrew
reports that the fix for PR gdb/18653 made GDB useless if you preload
libSegFault.so, because GDB internal-errors on startup:

 $ LD_PRELOAD=libSegFault.so gdb
 src/gdb/common/signals-state-save-restore.c:64: internal-error: unexpected signal handler
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Aborted (core dumped)
 $

The internal error comes from the code saving the signal dispositions
inherited from gdb's parent:

 (top-gdb) bt
 #0  0x000000000056b001 in internal_error(char const*, int, char const*, ...) (file=0xaf5f38 "src/gdb/common/signals-state-save-restore.c", line=64, fmt=0xaf5f18 "unexpected signal handler") at src/gdb/common/errors.c:54
 #1  0x00000000005752c9 in save_original_signals_state() () at src/gdb/common/signals-state-save-restore.c:64
 #2  0x00000000007425de in captured_main_1(captured_main_args*) (context=0x7fffffffd860)
     at src/gdb/main.c:509
 #3  0x0000000000743622 in captured_main(void*) (data=0x7fffffffd860) at src/gdb/main.c:1145
 During symbol reading, cannot get low and high bounds for subprogram DIE at 24065.
 #4  0x00000000007436f9 in gdb_main(captured_main_args*) (args=0x7fffffffd860) at src/gdb/main.c:1171
 #5  0x0000000000413acd in main(int, char**) (argc=1, argv=0x7fffffffd968) at src/gdb/gdb.c:32

This commit downgrades the internal error to a warning.  You'll get
instead:

~~~
 $ LD_PRELOAD=libSegFault.so gdb
 warning: Found custom handler for signal 11 (Segmentation fault) preinstalled.
 Some signal dispositions inherited from the environment (SIG_DFL/SIG_IGN)
 won't be propagated to spawned programs.
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 (gdb)
~~~

This also moves the location where save_original_signals_state is
called a bit further below (to after option processing), so that "-q"
disables the warning:

~~~
 $ LD_PRELOAD=libSegFault.so gdb -q
 (gdb)
~~~

New testcase included.

gdb/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	PR gdb/18653
	* common/signals-state-save-restore.c
	(save_original_signals_state): New parameter 'quiet'.  Warn if we
	find a custom handler preinstalled, instead of internal erroring.
	But only warn if !quiet.
	* common/signals-state-save-restore.h
	(save_original_signals_state): New parameter 'quiet'.
	* main.c (captured_main_1): Move save_original_signals_state call
	after option handling, and pass QUIET.

gdb/gdbserver/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	PR gdb/18653
	* server.c (captured_main): Pass quiet=false to
	save_original_signals_state.

gdb/testsuite/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	PR gdb/18653
	* gdb.base/libsegfault.exp: New.