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/util/utf8.c

@@ -0,0 +1,62 @@
+#include "util/utf8.h"
+
+#define UTF8_CONT_MSK 0b11000000
+#define UTF8_CONT_VAL 0b10000000
+bool utf8_iscont(char byte) {
+  return (byte & UTF8_CONT_MSK) == UTF8_CONT_VAL;
+}
+
+size_t utf8len(const char* str) {
+  size_t len = 0;
+  while (*str != '\0') {
+    if (!utf8_iscont(*(str++))) len++;
+  }
+
+  return len;
+}
+
+size_t utf8len_until(const char* str, const char* until) {
+  size_t len = 0;
+  while (str < until) {
+    if (!utf8_iscont(*(str++))) len++;
+  }
+
+  return len;
+}
+
+size_t utf8trunc(char* str, size_t n) {
+  size_t bytes = 0;
+  while (true) {
+    if (str[bytes] == '\0') break;
+    if (utf8_iscont(str[bytes])) {
+      bytes++;
+      continue;
+    }
+    if (n == 0) {
+      str[bytes] = '\0';
+      break;
+    }
+    bytes++;
+    n--;
+  }
+  return bytes;
+}
+
+const char* utf8back(const char* str) {
+  while (utf8_iscont(*(--str))) {
+  }
+  return str;
+}
+const char* utf8seek(const char* str) {
+  while (utf8_iscont(*(++str))) {
+  }
+  return str;
+}
+
+const char* utf8seekn(const char* str, size_t n) {
+  while (n > 0 && *str != '\0') {
+    str = utf8seek(str);
+    n--;
+  }
+  return str;
+}