Rename _doprnt -> _bfd_doprnt to fix DJGPP bfd build (and likely others)

Rename _doprnt -> _bfd_doprnt to fix DJGPP bfd build (and likely others)

Building bfd/ for DJGPP, I see:

 ../../src/bfd/bfd.c:642:1: error: conflicting types for '_doprnt'
  _doprnt (FILE *stream, const char *format, va_list ap)
  ^~~~~~~
 In file included from ../../src/bfd/sysdep.h:37:0,
		  from ../../src/bfd/bfd.c:367:
 /usr/lib64/gcc/i586-pc-msdosdjgpp/6.1.0/include-fixed/stdio.h:171:5: note: previous declaration of '_doprnt' was here
  int _doprnt(const char *_fmt, va_list _args, FILE *_f);
      ^~~~~~~

Possible fixes the build on other systems too -- _doprnt was/is an
exported symbol on old Unix/BSD systems too.

bfd/ChangeLog:
2017-10-11  Pedro Alves  <palves@redhat.com>

	* bfd.c (_doprnt): Rename to ...
	(_bfd_doprnt): ... this.
	(error_handler_internal): Adjust.