Use `errno_t` in all uspace and kernel code.

Use `errno_t` in all uspace and kernel code.

Change type of every variable, parameter and return value that holds an
<errno.h> constant to either `errno_t` (the usual case), or `sys_errno_t`
(some places in kernel). This is for the purpose of self-documentation,
as well as for type-checking with a bit of type definition hackery.

Although this is a massive commit, it is a simple text replacement, and thus
is very easy to verify. Simply do the following:

```
git checkout <this commit's hash>
git reset HEAD^
git add .
tools/srepl '\berrno_t\b' int
git add .
tools/srepl '\bsys_errno_t\b' sysarg_t
git reset
git diff
```

While this doesn't ensure that the replacements are correct, it does ensure
that the commit doesn't do anything except those replacements. Since errno_t
is typedef'd to int in the usual case (and sys_errno_t to sysarg_t), even if
incorrect, this commit cannot change behavior.