Nginx setup: Wordpress installation in root + Wordpress Multisite installation in subfolder?

First off, I just setup a debian server with LEMP. I have a very special setup, which is one Wordpress installation in the root folder, and a Wordpress Multisite network which is installed in a subfolder called “/u”. The multisite network is served on subfolders too, not subdomains.

The current server is using Apache, which is setup this way. Main (single site, in root):

<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteBase /
  Redirect /login /u/login
  Redirect /register /u/register
  Redirect /link /u/link
  Redirect /link-tv /u/link-tv
  Redirect /wp-login.php /u/wp-login.php
</IfModule>

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Multisite (subdir network, install in “/u”)

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteBase /u/
    RewriteRule ^(([_0-9a-zA-Z-]+/)?)postpass/?$ /u/wp-login.php?action=postpass [QSA,L]
    RewriteRule ^(([_0-9a-zA-Z-]+/)?)logout/?$ /u/wp-login.php?action=logout [QSA,L]
    RewriteRule ^(([_0-9a-zA-Z-]+/)?)lostpassword/?$ /u/wp-login.php?action=lostpassword [QSA,L]
    RewriteRule ^(([_0-9a-zA-Z-]+/)?)resetpassword/?$ /u/wp-login.php?action=resetpass [QSA,L]
    RewriteRule ^(([_0-9a-zA-Z-]+/)?)signup/?$ /u/wp-login.php?action=register [QSA,L]
    RewriteRule ^(([_0-9a-zA-Z-]+/)?)login/?$ /u/wp-login.php [QSA,L]
    RewriteRule ^(([_0-9a-zA-Z-]+/)?)dashboard/?$ /u/wp-login.php [QSA,L]
</IfModule>


RewriteEngine On
RewriteBase /u
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

This Apache setup works great, but how do I convert this to work on Nginx?

All help is appreciated! Thanks!