BUG/MEDIUM: workaround an eglibc bug which truncates the pidfiles when nbproc > 1

BUG/MEDIUM: workaround an eglibc bug which truncates the pidfiles when nbproc > 1

Thomas Heil reported that when using nbproc > 1, his pidfiles were
regularly truncated. The issue could be tracked down to the presence
of a call to lseek(pidfile, 0, SEEK_SET) just before the close() call
in the children, resulting in the file being truncated by the children
while the parent was feeding it. This unexpected lseek() is transparently
performed by fclose().

Since there is no way to have the file automatically closed during the
fork, the only solution is to bypass the libc and use open/write/close
instead of fprintf() and fclose().

The issue was observed on eglibc 2.15.