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>

src/keys.c

@@ -0,0 +1,77 @@
+#include <errno.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "keys.h"
+#include "macros.h"
+#include "ui.h"
+
+static int selret_magic();
+
+int find_keyname(enum Keys* at, const char* name) {
+  for (size_t i = 0; i < LEN(KEY_MAPPINGS); i++) {
+    if (strcmp(KEY_NAMES[i], name) == 0) {
+      *at = (enum Keys)i;
+      return 0;
+    }
+  }
+
+  return -1;
+}
+
+struct option_keys find_ansi(const char* seq) {
+  for (size_t i = 0; i < LEN(KEY_MAPPINGS); i++) {
+    struct key_mapping mapping = KEY_MAPPINGS[i];
+    for (size_t j = 0; mapping.sequences[j] != NULL; j++) {
+      if (strcmp(mapping.sequences[j], seq) == 0) {
+        return (struct option_keys){
+            .is_some = true,
+            .key = (enum Keys)i,
+        };
+      }
+    }
+  }
+  return (struct option_keys){.is_some = false};
+}
+
+void read_press(uint8_t* length, char* out) {
+  *length = 0;
+
+  while (true) {
+    if (read(STDIN_FILENO, &out[(*length)++], 1) != 1) {
+      print_errno("read error");
+      sleep(3);
+      exit(1);
+    }
+    int selret = selret_magic();
+    if (selret == -1) {
+      print_errno("selret error");
+    } else if (selret != 1) {
+      out[*length] = '\0';
+      return;
+    }
+  }
+}
+
+bool read_press_nb(uint8_t* length, char* out, struct timeval* tv) {
+  fd_set fds;
+  FD_ZERO(&fds);
+  FD_SET(STDIN_FILENO, &fds);
+  errno = 0;
+  int ret = select(STDIN_FILENO + 1, &fds, NULL, NULL, tv);
+  if (errno || ret <= 0) return false;
+
+  read_press(length, out);
+  return true;
+}
+
+// https://stackoverflow.com/a/48040042
+static int selret_magic() {
+  fd_set set;
+  struct timeval timeout;
+  FD_ZERO(&set);
+  FD_SET(STDIN_FILENO, &set);
+  timeout.tv_sec = 0;
+  timeout.tv_usec = 0;
+  return select(1, &set, NULL, NULL, &timeout);
+}