chcon: use security_check_context() for context validation

chcon: use security_check_context() for context validation

context_new() and _free() are used for checking validity of a
specified context.  libselinux provides security_check_context
for this purpose so use it.

Note that context_new() can fail for a valid context - e.g. ENOMEM.

* src/chcon.c (main): Use security_check_context().