ctdb-locking: Use vfork instead of fork to exec helpers

ctdb-locking: Use vfork instead of fork to exec helpers

There is a significant overhead using fork() over vfork(), specially
when the child process execs a helper.  The overhead is in memory space
and time.

    # strace -c ./test_fork 1024 200
    count=1024, size=204800, total=200M
    failed fork=0
    time for fork() = 4879.597000 us
    % time     seconds  usecs/call     calls    errors syscall
    ------ ----------- ----------- --------- --------- ----------------
    100.00    4.543321        3304      1375       375 clone
      0.00    0.000071           0      1033           mmap
      0.00    0.000000           0         1           read
      0.00    0.000000           0         3           write
      0.00    0.000000           0         2           open
      0.00    0.000000           0         2           close
      0.00    0.000000           0         3           fstat
      0.00    0.000000           0         3           mprotect
      0.00    0.000000           0         1           munmap
      0.00    0.000000           0         3           brk
      0.00    0.000000           0         1         1 access
      0.00    0.000000           0         1           execve
      0.00    0.000000           0         1           arch_prctl
    ------ ----------- ----------- --------- --------- ----------------
    100.00    4.543392                  2429       376 total

    # strace -c ./test_vfork 1024 200
    count=1024, size=204800, total=200M
    failed fork=0
    time for fork() = 82.041000 us
    % time     seconds  usecs/call     calls    errors syscall
    ------ ----------- ----------- --------- --------- ----------------
     96.47    0.001204           1      1000           vfork
      3.53    0.000044           0      1033           mmap
      0.00    0.000000           0         1           read
      0.00    0.000000           0         3           write
      0.00    0.000000           0         2           open
      0.00    0.000000           0         2           close
      0.00    0.000000           0         3           fstat
      0.00    0.000000           0         3           mprotect
      0.00    0.000000           0         1           munmap
      0.00    0.000000           0         3           brk
      0.00    0.000000           0         1         1 access
      0.00    0.000000           0         1           execve
      0.00    0.000000           0         1           arch_prctl
    ------ ----------- ----------- --------- --------- ----------------
    100.00    0.001248                  2054         1 total

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Michael Adam <obnox@samba.org>