Use of #define _HEADERNAME_H_

[Mark Doliner]

Dear developers,

It has come to my attention that we shouldn't be using names that start with
an underscore followed by a capital letter.  They're reserved for use by the
compiler and system libraries.  See the paragraph near the top of
http://www.gnu.org/software/libc/manual/html_node/Reserved-Names.html which
begins with "In addition."

So I'm proposing that we change

#ifndef _PURPLE_ACCOUNT_H_
#define _PURPLE_ACCOUNT_H_
to
#ifndef PURPLE_ACCOUNT_H
#define PURPLE_ACCOUNT_H

in all our header files.  I feel that that does not break minor or micro API
compatibility.  Any objections?

Love,
Mark