[perl #87336] lc/uc(first) fail to taint the returned string

[perl #87336] lc/uc(first) fail to taint the returned string

This bug was caused by change 28011 (ec9af7d), which stopped pp_lc
from using sv_setsv_flags, thereby bypassing these two lines at the
end of that function:
    if (SvTAINTED(sstr))
	SvTAINT(dstr);

Change 28012 (6730619) did exactly the same thing to pp_uc.

28013 (d54190f) broke ucfirst and lcfirst.

This commit simply puts that taint logic at the end of the pp_*
functions.