[LIBS] Add strspn, strcspn and strtok_r string operations.

Add 3 standard string operation for string manipulation.

Signed-off-by: Himanshu Chauhan <hschauhan@nulltrace.org>
[COMMANDS] Add support for execution of a batch/script file.

This patch adds a sub-command to vfs command for running
set of Xvisor commands from a batch/script file. The batch
file is a text file with set of Xvisor commands. The lines
that start with '#' are treated as comment and not executed.

Signed-off-by: Himanshu Chauhan <hschauhan@nulltrace.org>