implement POSIX confstr() in System/Posix/Unistd.hsc

[clint]

patch implementing confstr()

[simonmar]

confstr is fine, but most of those values (e.g. _CS_LFS_CFLAGS) are non-portable as far as I can tell. The only portable one is _CS_PATH.

See e.g. the POSIX pages that list the standard defines:  http://pubs.opengroup.org/onlinepubs/009695399/basedefs/unistd.h.html and  http://pubs.opengroup.org/onlinepubs/009695399/functions/confstr.html

[clint]

So can they be included conditionally?

[simonmar]

In general it's not OK to have platform-conditional APIs in the unix package. Typically a missing function will throw an exception (e.g. getSlaveTerminalName), or fall back to a portable version (e.g. unsetEnv). Where there are constants or symbols that need to be conditional, we use a .Exts module (see e.g. System.Posix.Signals.Exts).