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>

debug/lidm-debug

@@ -0,0 +1,9 @@
+type            = process
+command         = /bin/sh -c "LIDM_LOG=/tmp/lidm.debug /sbin/agetty tty8 linux-c -n -l /usr/local/bin/lidm -o 8"
+restart         = true
+depends-on      = login.target
+termsignal      = HUP
+smooth-recovery = true
+inittab-id      = 8
+inittab-line    = tty8
+log-type        = buffer