Fix uses of deprecated SELinux security_context_t

Fix uses of deprecated SELinux security_context_t

SELinux has used 'char *' in place of its typedef
'security_context_t' since 2014 (v2.3) because the latter was
inconvenient to use when paired with the 'const' qualifier.  The
typedef has been kept around for compatibility with legacy callers,
but it is deprecated in SELinux v3.1.  See the following URL for the
relevant announcement:
https://lore.kernel.org/selinux/20200710162034.GC1768200@localhost.localdomain/

* src/fileio.c (Fcopy_file, Ffile_selinux_context)
(Fset_file_selinux_context) [HAVE_LIBSELINUX]: Replace deprecated
'security_context_t' typedef with the equivalent 'char *'.