There was no particular reason to use a do-while loop.

See also: commit 10968399

This works when opening files, but more importantly it also works when
piping to stdin at startup. For example:

    git log -p HEAD~8..HEAD | dte
    git stash list -p | dte

See also:

* Commit f75e62f2
* Commit 03900b66

See also: commit 03900b66

Quick usage example:

    git log -p HEAD~8..HEAD | dte -c 'set filetype gitlog'

> src/util/strtonum.h:43:8: warning: function 'buf_parse_uintmax'
>  has a definition with different parameter names
>  [readability-inconsistent-declaration-parameter-name]
> src/util/strtonum.h:44:8: warning: function 'buf_parse_ulong' ...
> src/util/strtonum.h:45:8: warning: function 'buf_parse_uint' ...

See the added comments for more details.

Quick usage example:

    git stash list -p | dte -c 'set filetype gitstash'

...or alternatively, from inside the editor:

    open -t; set filetype gitstash; pipe-from git stash list -p

These are probably of limited use, but the implementation is simple
enough and there's no particular reason to not support them.

Note: this has been left undocumented for now, which implies it may
still receive breaking changes in the future.

Also add a new `-M` flag to both commands, which makes them never
move the cursor (thus becoming no-ops when at the scroll margins).

Previously there was an inconsistency where a non-zero `scroll-margin`
would produce the no-op behavior, whereas with `set scroll-margin 0`
the cursor would always be moved when at the margins. This commit makes
these 2 commands behave the same way, regardless of the `scroll-margin`
value.

The `scroll-pgup` and `scroll-pgdown` commands will be amended in a
similar fashion, in a follow-up commit.

Note: this feature has been left undocumented for now, which implies
it may still receive breaking changes in the future.

The passed in value is always taken from `GlobalOptions::scroll_margin`,
so we may as well just access that via `Window::editor::options` from
within the function.

For the HTML version of the `dterc(5)` man page.

Instead of `EC_`, which is also used in `src/util/exitcode.h`.

This matches the behaviour of `insert_paste()` (i.e. pastes from the
terminal) for these 2 modes.

This commit also adds a new `paste -n` flag, which replaces newlines
with semicolons instead.

Note: the `-n` flag has been left undocumented for now, which implies
it may still receive breaking changes in the future.

This makes these previously "mutually exlusive" sets of flags now behave
as "last one wins", which can sometimes be useful in aliases and removes
some unnecessary error cases.

This shrinks the generated code by 8 instructions (from 48 to 40) on
x86_64/Linux and (of course) replaces the `CALL` with a `JMP`.

Instead of producing incorrect output for `build/gen/builtin-config.h`,
which later causes obscure error messages when dte starts.

This commit also removes the `cmdline_x` member from `EditorState`,
since it can just as well be a local variable and has no reason to
persist beyond each call to `update_screen()`.