Move strlcat, strlcpy, and strnlen

Move strlcat() and strlcpy() from .c source files in to the libspl
string.h header. By changing these compatibility functions to static
inline functions they can included as needed without requiring linking
with the libspl.so library.

Remove strnlen() which is barely used in the source, and has been
provided by glibc since v2.10.

Finally, convert four instances of strncpy() to strlcpy() in
libzfs_input_check.c which were causing build warnings when compiling
with gcc 8.2.1. For example:

  libzfs_input_check.c: In function ‘zfs_destroy’:
  libzfs_input_check.c:651:9: error: ‘strncpy’ specified bound \
      4096 equals destination size [-Werror=stringop-truncation]
    (void) strncpy(zc.zc_name, dataset, sizeof (zc.zc_name));
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8116