getrandom: print string as hex-escaped

getrandom: print string as hex-escaped

Since there is no reason to interpret the value returned by getrandom as
ASCII string, it makes sense to always print it as a hex-escaped string.

* getrandom.c (SYS_FUNC(getrandom)): Use printstr_ex instead
of printstrn, set QUOTE_FORCE_HEX in user_style parameter in order
to force hex-escaped string formatting.
* tests/getrandom.test: Remove no longer needed -xx flag as the string
is always printed in hexadecimal format now.

Suggested-by: JingPiao Chen <chenjingpiao@gmail.com>