get_maintainer: correctly parse UTF-8 encoded names in files

While the script correctly extracts UTF-8 encoded names from the
MAINTAINERS file, the regular expressions damage my name when parsing
from .yaml files.  Fix this by replacing the Latin-1-compatible regular
expressions with the unicode property matcher \p{L}, which matches on
any letter according to the Unicode General Category of letters.

The proposed solution only works if the script uses proper string
encoding from the outset, so instruct Perl to unconditionally open all
files with UTF-8 encoding.  This should be safe, as the entire source
tree is either UTF-8 or ASCII encoded anyway.  See [1] for a detailed
analysis.

Furthermore, to prevent the \w expression from matching non-ASCII when
checking for whether a name should be escaped with quotes, add the /a
flag to the regular expression.  The escaping logic was duplicated in
two places, so it has been factored out into its own function.

The original issue was also identified on the tools mailing list [2].
This should solve the observed side effects there as well.

Link: https://lore.kernel.org/all/dzn6uco4c45oaa3ia4u37uo5mlt33obecv7gghj2l756fr4hdh@mt3cprft3tmq/ [1]
Link: https://lore.kernel.org/tools/20230726-gush-slouching-a5cd41@meerkat/

 [2]
Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>