Add support for using pledge() on OpenBSD

[fcambus]

This diff adds support for OpenBSD pledge(2).

First time I tweak some autotools related files so I hope I didn't break anything :D I'm not sure if unistd.h should be added to AC_CHECK_HEADERS in configure.ac, and tests without it seems to run fine, however it might be better to have it to be on the safe side?

Tested on OpenBSD, NetBSD and Mac OS X.

[jpmens]

$ uname -a
OpenBSD ot-make-obsd58 5.8 GENERIC#1170 amd64

$ ./configure 
...
checking for unistd.h... yes
...
checking for pledge... no

If I change #if HAVE_PLEDGE to #if 1 in jo.c:

# make
  CCLD     jo
jo.o: In function `main':
/root/jo/jo-0.9/jo.c:368: undefined reference to `pledge'
collect2: ld returned 1 exit status

[jpmens]

There's no pledge(2) page on 5.8, so am I barking up the wrong tree?

[jpmens]

Merged, thank you!

[jpmens]

OpenBSD 5.9

checking for pledge... yes

If I add, say, a call to system("ls"); just after the pledge() call, I get

# ./jo
Abort trap (core dumped)

Very nice.