g_utf8_validate_len doesn't exist until glib 2.60, so just
use g_utf8_validate.  It does the same thing when the length parameter
is unsigned. Fixes CentOS 7.

Add BASE_SHOW_UTF_8_PRINTABLE and related function tvb_utf_8_isprint
for supporting fields of bytes that are "maybe UTF-8" (default or
SHOULD be UTF-8 but could be something else, with no encoding indicator),
such as SSID fields in IEEE 802.11 (See #16208), certain OctetString
fields in Diameter or PFCP, and other places where
BASE_SHOW_ASCII_PRINTABLE is currently used. Fix #5307

This fixes an error due to memcpy not being declared.  Closes #17911.