[add] Use 16MB static buffer for files

Basically no performance improvement when running unbuffered:
time ./ngp_perf int xxx
Found 61340 files, 480476 lines

real	0m50.552s
user	0m1.668s
sys	0m10.210s

time ./ngp_perf_sb int xxx
Found 61340 files, 480476 lines

real	0m50.115s
user	0m1.653s
sys	0m9.297s

But very substantial speedup when running from cache:
time ./ngp_perf int xxx
Found 61340 files, 480476 lines

real	0m2.915s
user	0m0.897s
sys	0m1.997s

time ./ngp_perf_sb int xxx
Found 61340 files, 480476 lines

real	0m2.378s
user	0m0.803s
sys	0m1.539s

[fix] typo error in subsearch mode selection