fixes: improve login environment handling (#114)

* chore(organization): allow nested header and c files

* feat: add shell login "middleware" & etc:
  * lidm now calls `bash` (or other shells, depends on PACKAGE cfg) in
  login mode as a session wrapper to source most env (can be disabled)
  * this fixes a lot of env problems with all `/etc/profile` and more

Extra:
* implemented a musl compatible version of `execvpe` and now lidm should
  search for PATH everywhere it needs to
* `search_path` now also checks if the found binary is properly
  executable
* lidm now uses `confstr` for a decent PATH default if none is found
* logs are unbuffered for cases where debug logs appear empty (exit
  without handlers moment)

* chore: one-time evaluate certain makefile vars

---------

Co-authored-by: grialion <48643945+grialion@users.noreply.github.com>

docs/INSTALL.md

@@ -50,7 +50,6 @@ It is recommended to disable any other display managers that are still enabled:
 touch /etc/sv/lightdm/down # replace lightdm with your previous display manager (e.g., sddm, gdm)
 ```
 
-
 ## Fedora
 
 Thanks to @KernelFreeze there's a COPR repo available at <https://copr.fedorainfracloud.org/coprs/celestelove/lidm/> to install lidm. (<https://github.com/javalsai/lidm/discussions/92>)
@@ -59,9 +58,9 @@ Thanks to @KernelFreeze there's a COPR repo available at <https://copr.fedorainf
 
 There are several [AUR packages](https://aur.archlinux.org/packages?K=lidm&SeB=n) for most things. You only need to install one of the following:
 
-* `lidm`
-* `lidm-bin`
-* `lidm-git`
+- `lidm`
+- `lidm-bin`
+- `lidm-git`
 
 Each of those depend on `lidm-service` which is provided by `lidm-systemd` and `lidm-dinit` (you'll have to pick one). I maintain both but I'm open to anyone that actually uses systemd to take the first one.
 

docs/PACKAGERS.md

@@ -29,6 +29,7 @@ The list of possible `#define`s is:
 | `SESSIONS_WAYLAND`         | `"/usr/share/wayland-sessions"`       |                                                                            | No                       |
 | `SESSIONS_WAYLAND_LOCAL`   | `"/usr/local/share/wayland-sessions"` |                                                                            | No                       |
 | `LIDM_CONF_PATH`           | `"/etc/lidm.ini"`                     | Path of the default configuration.                                         | Yes (`LIDM_CONF`)        |
+| `LOGIN_SHELL`              | `"bash"`                              | Login shell name.                                                          | No                       |
 
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