include stdlib.h and stop redeclaring malloc, free in alloc.c

[alanpost]

This change is necessary for -Wall -Werror, to prevent the following:

./compile alloc.c
alloc.c:3:14: error: conflicting types for built-in function ‘malloc’ [-Werror]
extern char *malloc();
^~~~~~

Which cannot be turned off without also turning off -Werror.