MinGW already has a redefine for ssize_t.

Previously os_getperms() returned -1 for any error condition, it
now returns the libuv error code (as returned by os_stat()). This
allows checking for error conditions without relying on errno
(which not available in Windows).

The only case where the errno value from os_getperms() was being used
was in readfile() to set the new-file flag - replaced the errno check
with UV_ENOENT.

Instead of returning bool from os_stat return the actual libuv return code.
This function is static and used internally in nvim/os/fs.c it should not
impact the rest of the API. This is a first step to change other fs functions.

In windows libuv does not return -errno, instead it uses negative
error codes e.g. UV_ENOENT. This commit changes the comments in os_*
functions to reflect this.

This is the last direct getenv() call in the tree (besides the one in
os_getenv()); most of the work was already done in [1].

[1]: 412d246be71bd99cb4edde4e6f984b0b0d91bcd9

Some more fixes to XDG code

The assertion now considers the case where events are disabled.