Introduce shell_expand

This performs shell expansion without field splitting. This fixes issues with
filenames containing spaces.

This commit also cleans up complicated logic to join fields and to set
XDG_CONFIG_HOME.

include/shexp.h

@@ -0,0 +1,14 @@
+#ifndef _SWAY_SHEXP_H
+#define _SWAY_SHEXP_H
+
+#include <stdbool.h>
+
+/**
+ * Takes a pointer to a string and reallocates it with the result of its shell
+ * expansion. Undefined environment variables are considered as errors. Returns
+ * true if it succeeds, otherwise returns false, leaving the original string
+ * unchanged.
+ */
+bool shell_expand(char **path);
+
+#endif