Instead of checking Fcall.data==nil, check Fcall.count==0. The former check always fails after `gcc -O2` optimizations (gcc version 8.3.0). Also fix an out-of-bound read detected by valgrind: ``` ==31162== Invalid read of size 1 ==31162== at 0x11005E: morerules (rules.c:739) ==31162== by 0x110254: writerules (rules.c:775) ==31162== by 0x10D2FE: fsyswrite (fsys.c:848) ==31162== by 0x10C304: fsysproc (fsys.c:248) ==31162== by 0x112E8C: threadstart (thread.c:96) ==31162== by 0x4A682BF: ??? (in /usr/lib/libc-2.29.so) ==31162== Address 0x4ea984a is 0 bytes after a block of size 250 alloc'd ==31162== at 0x483AD7B: realloc (vg_replace_malloc.c:826) ==31162== by 0x1196F3: p9realloc (malloc.c:53) ==31162== by 0x10BDFD: erealloc (plumber.c:124) ==31162== by 0x10FCD9: concat (rules.c:642) ==31162== by 0x10FCD9: concat (rules.c:635) ==31162== by 0x110230: writerules (rules.c:773) ==31162== by 0x10D2FE: fsyswrite (fsys.c:848) ==31162== by 0x10C304: fsysproc (fsys.c:248) ==31162== by 0x112E8C: threadstart (thread.c:96) ==31162== by 0x4A682BF: ??? (in /usr/lib/libc-2.29.so) ``` Fixes #256

R=rsc http://codereview.appspot.com/6210059

More changes.

Add debugging to fdwait. Rewrite getcallerpc on PowerMac to be correct.