send-mail: improve checks for valid_fqdn

The current implementation of a valid Fully Qualified Domain Name
is not that strict. It just checks whether it has a dot (.) and
if using macOS, it should not end with .local. As per RFC1035[1],
from what I understood, the following checks need to be done:

- The domain must contain atleast one dot
- Each label (separated by dots) must be 1-63 characters long
- Labels must start and end with an alphanumeric character
- Labels can contain alphanumeric characters and hyphens

Here are some examples of valid and invalid labels:

'example.com',          # Valid
'sub.example.com',      # Valid
'my-domain.org',        # Valid
'localhost',            # Invalid (no dot)
'MacBook..',            # Invalid (double dots)
'-example.com',         # Invalid (starts with a hyphen)
'example-.com',         # Invalid (ends with a hyphen)
'example..com',         # Invalid (double dots)
'example',              # Invalid (no TLD)
'example.local',        # Invalid on macOS
'valid-domain.co.uk',   # Valid
'123.example.com',      # Valid
'example.com.',         # Invalid (trailing dot)
'toolonglabeltoolonglabeltoolonglabeltoolonglabeltoolonglabeltoolonglabel.com', # Invalid (label > 63 chars)

Due to current implementation, I was not able to send emails from
Ubuntu. Upon debugging, I found that the SMTP domain being passed
to Outlook's servers was not valid.

Net::SMTP=GLOB(0x5db4351225f8)>>> EHLO MacBook..
Net::SMTP=GLOB(0x5db4351225f8)<<< 501 5.5.4 Invalid domain name
Net::SMTP=GLOB(0x5db4351225f8)>>> HELO MacBook..

Notice that an invalid domain name "MacBook.." is sent by git-send-email.
We have a fallback code that checks output from Net::Domain::domainname()
or asking domain method of an Net::SMTP instance to detect a misconfigured
hostname and replace it with fallback "localhost.localdomain", but the
valid_fqdn apparently is failing to say "MacBook.." is not a valid fqdn.

With this patch, the rule used in valid_fqdn is tightened, the beginning
part of the SMTP exchange looked like this:

Net::SMTP=GLOB(0x58c8af71e930)>>> EHLO localhost.localdomain
Net::SMTP=GLOB(0x58c8af71e930)<<< 250-PN4P287CA0064.outlook.office365.com Hello

[1]: https://datatracker.ietf.org/doc/html/rfc1035

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: AdityaGarg8

git-send-email.perl

@@ -1354,7 +1354,9 @@ sub process_address_list {
 
 sub valid_fqdn {
 	my $domain = shift;
-	return defined $domain && !($^O eq 'darwin' && $domain =~ /\.local$/) && $domain =~ /\./;
+	my $subdomain = '(?!-)[A-Za-z0-9-]{1,63}(?<!-)';
+	return defined $domain && !($^O eq 'darwin' && $domain =~ /\.local$/)
+		&& $domain  =~ /^$subdomain(?:\.$subdomain)*$/;
 }
 
 sub maildomain_net {