Move pre_check for ASCII string out of PRE(sys_prctl)

Move pre_check for ASCII string out of PRE(sys_prctl)

The sys_prctl wrapper with PR_SET_NAME option reads an ASCII string passed
as its second argument. This string is supposed to be shorter than a given
limit. As the actual length of the string is unknown, the PRE() wrapper
performs a number of checks on it, including, in worst case, trying to
dereference it byte by byte.

To avoid re-implementing all this logic for other wrappers that could
need it, get the string processing out of the wrapper and move it to a
static function. Note that passing tid as an argument to the function is
required for macros PRE_MEM_RASCIIZ and PRE_MEM_READ to work properly.