Remove prototypes for getenv. Use stdlib.h. (#122)

This also removes some casts to `char *` that were present but
not needed, since `getenv()` is defined to return `char *`.

src/initkbd.c

@@ -12,6 +12,7 @@
 
 #include <errno.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
 #ifndef DOS
@@ -383,8 +384,6 @@ void seteventmask(struct inputmask *eventmask)
 }
 #endif /* SUNDISPLAY */
 
-char *getenv(); /*  ---- external entry points --------*/
-
 #define MIN_KEYTYPE 3
 #define KB_AS3000J (7 + MIN_KEYTYPE)
 #define KB_RS6000 (8 + MIN_KEYTYPE) /* TODO: Can we remove this? */