exile.h: Move definitions to new file exile.c

Especially with exile_launch(), we will be included
from more than one translation unit. Thus, ODR becomes
a headache now.

So move definitions to exile.c.

test.c

@@ -489,6 +489,7 @@ int test_no_new_fds()
 
 }
 
+extern int mkpath(const char *p, mode_t mode, int baseisfile);
 int test_mkpath()
 {
 	system("rm -rf /tmp/.exile.h/");