Fixes compiler warning: implicit declaration of function 'isalnum'
Fixes compiler warning: control reaches end of non-void function
The header file limits.h was being included twice
Turns on gcc warnings (CFLAGS += -Wall -Wextra)
Defines _GNU_SOURCE (required by the unshare syscall)
Change cgroup() return type from int to void (avoids a gcc warning)
Merge pull request #218 from jpoliv/master

Fixes a couple of compiler warnings