insert_with_markers is not threadsafe

[strings28]

From wp-admin/includes/misc.php the function insert_with_markers may be called multiple times on a busy server and if the htaccess is already in the process of being written it is possible that two PHP threads could attempt to write to it causing corruption such as the following:

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

# END WordPress
s

Notice the dangling 's' at the last line

A proposed patch which creates a lock file