Optimize libtar extract

[JonathonReinhart]

This updates tar_extract_regfile() to make a single write() syscall, rather than writing 512 bytes at a time.

Closes #130

strace -c ./id-dev.sx 
uid=1000(jreinhart) gid=1000(jreinhart) groups=1000(jreinhart),...
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 31.61    0.000325          46         7           write
 17.12    0.000176           8        20         7 unlink
  7.59    0.000078           1        51           open
  6.23    0.000064           1        63           close
  6.03    0.000062           0        74           read
  5.54    0.000057           0        81           fcntl
  3.31    0.000034          34         1           rmdir
  3.21    0.000033           2        12        12 connect
  3.02    0.000031           5         6           symlink
  2.53    0.000026           5         5           munmap
  2.24    0.000023           7         3           madvise
  2.14    0.000022           0        27        26 mkdir
  1.85    0.000019           0        28           lseek
  1.85    0.000019           1        12           socket
  1.36    0.000014           2         7           utimensat
  0.97    0.000010           1         7           chmod
  0.68    0.000007           0        13           geteuid
  0.68    0.000007           3         2           getdents64
  0.49    0.000005           0         6           mmap
  0.49    0.000005           5         1           fork
  0.29    0.000003           0        14           lstat
  0.29    0.000003           0         4           rt_sigaction
  0.19    0.000002           2         1           wait4
  0.10    0.000001           1         1           stat
  0.10    0.000001           1         1           access
  0.10    0.000001           1         1           readlink
  0.00    0.000000           0         3           fstat
  0.00    0.000000           0         6           brk
  0.00    0.000000           0         3           rt_sigprocmask
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         1           arch_prctl
  0.00    0.000000           0         1           set_tid_address
------ ----------- ----------- --------- --------- ----------------
100.00    0.001028                   463        45 total

My unscientific test shows the total write going:

• Before: 5517 calls, total 0.009341 sec
• After: 7 calls, total 0.000325 sec (-96.5% 🎉)

[JonathonReinhart]

Travis is being a douchebag (#124) and not showing the checks, but the tests all passed.

[Ajckxska]

1