Introduce common/errors.h

Introduce common/errors.h

This introduces common/errors.h.  This holds some error- and warning-
related declarations that can be used by the code in common, nat and
target.  Some of the declared functions must be provided by the client
as documented by the header file comments.

gdb/ChangeLog:

	* common/errors.h: New file.
	* common/errors.c: Likewise.
	* Makefile.in (SFILES): Add common/errors.c.
	(HFILES_NO_SRCDIR): Add common/errors.h.
	(COMMON_OBS): Add errors.o.
	(errors.o): New rule.
	* common/common-defs.h: Include errors.h.
	* utils.h (perror_with_name, error, verror, warning, vwarning):
	Don't declare.
	* common/common-utils.h: (malloc_failure, internal_error):
	Likewise.

gdb/gdbserver/ChangeLog:

	* Makefile.in (SFILES): Add common/errors.c.
	(OBS): Add errors.o.
	(IPA_OBS): Add errors-ipa.o.
	(errors.o): New rule.
	(errors-ipa.o): Likewise.
	* utils.h (perror_with_name, error, warning): Don't declare.
	* utils.c (warning): Renamed and rewritten as...
	(vwarning): New function.
	(error): Renamed and rewritten as...
	(verror): New function.
	(internal_error): Renamed and rewritten as...
	(internal_verror): New function.