get_maintainer: add email addresses from .yaml files

.yaml files can contain maintainer/author addresses and it seems unlikely
or unnecessary that individual MAINTAINER file section entries for each
.yaml file will be created.

So add the email addresses found in .yaml files to the default
get_maintainer output.

The email addresses are marked with "(in file)" when using the "--roles"
or "--rolestats" options.

Miscellanea:

o Change $file_emails to $email_file_emails to avoid visual
  naming conflicts with @file_emails

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: http://lkml.kernel.org/r/e85006456d9dbae55286c67ac5263668a72f5b58.1588022228.git.joe@perches.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>