syscalls: fix cwd-related stuff

include/exec_from_file.h

@@ -2,4 +2,4 @@
 #include "../include/elf.h"
 #include "../include/stdio.h"
 
-void exec_from_file(const char* filename);
+void exec_from_file(const char* filename, const char* dirname);

include/string.h

@@ -8,5 +8,7 @@ void* memcpy(void* __restrict, const void* __restrict, size_t);
 void* memmove(void*, const void*, size_t);
 void* memset(void*, int, size_t);
 size_t strlen(const char*);
+char *strcat(char *dest, const char *src);
+char *strcpy(char *dest, const char *src);
 
 #endif