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>

CHANGELOG.md

@@ -4,6 +4,12 @@
 <!-- By "very relevant" I mean big features or something manual packagers should know, like leftover files -->
 <!-- Once a release would be opened, group the last bunch of dangling changes, add release version as header and its date -->
 
+- lidm now calls bash for logging in and sourcing profile files, other shells can be configured
+- a good PATH default is fetched from `confstr`
+- debug logs are now unbuffered
+
+# 2.0.0
+
 - Most stuff (most of `/etc` being a notable exception) now installs to `/usr/local` by default, check [`docs/PACKAGERS.md`](./docs/PACKAGERS.md).
 - Added a changelog.
 - Finally add proper (experimental) xorg support.