uspace: make libudev optional

.. it's not available on debian kfreebsd.

Userspace PCI uses libudev to enumerate available PCI devices, and then memory
maps files in /sys to access the device.  Neither libudev nor this /sys interface
exist on debian kfreebsd.
rtapi: disable /proc for kernel 3.10+

Reportedly, classic kernel APIs we used for the /proc filesystem
have been removed in Linux 3.10.  If so, don't even try.

(untested)
hal: disable /proc for kernel 3.10+

Reportedly, classic kernel APIs we used for the /proc filesystem
have been removed in Linux 3.10.  If so, don't even try.

(untested)
rtapi_math: drop unneeded gcc version check

As documented at http://wiki.linuxcnc.org/cgi-bin/wiki.pl?MinimumSoftwareVersions
the oldest platform we're likely to target with linuxcnc 2.7 is Ubuntu 10.04
with gcc 4.4.
rtapi_math: Move inlines out of i386.h

.. otherwise the implementations are not available e.g., on x86_64 in kernel
space
fmin/fmax: Correct return value of fmin(finite, nan)

According to C99, in the case that one argmuent is nan, the other
argument is returned.  This case is likely not exercised by our code,
but someday it might be (nan/inf are only forbidden as a value on a pin
or signal; it's not forbidden internally to a realtime component)
build: -fstrict-overflow is redundant with -fwrapv

This is true in both gcc and clang, but clang goes so far as to issue
a warning about it.

Furthermore, even our minimum gcc version (4.4) has -fwrapv, so this
doesn't even need to be conditioned on the compiler accepting it.
pythonplugin: remove unused fields

clang detected that these fields were unused.  delete them.
taskclass: remove unused field

clang detects that this field is unused.
tp: Remove unused functions

clang detects that these functions are unused.
togl: cast to avoid warning

clang complains about the comparison between integer and pointer
emcmodule: Disable invalid offsetof warnings explicitly

"-Winvalid-offsetof" has been a g++ flag since 3.x, so we can use it
in all gcc versions; it is better than the previous hack, which
encountered C++11 narrowing diagnostics with clang 3.5
ladder: get rid of unused dbg_printf

.. the unused inline function was a warning in clang
halrmt: If you mean NULL, say NULL

clang warns about the assignment of a character literal to a pointer
nml: 'register' is a deprecated keyword

.. drop it
uspace: Don't call RtapiApp a struct

clang's silly about this one, it doesn't like later calling something
a 'class' when it was first declared with 'struct'.
rtapi: don't uselessly test if an unsigned is less than zero
Merge branch 'jepler/proposed-master'